diff --git a/google/cloud/translate_v3/services/translation_service/transports/base.py b/google/cloud/translate_v3/services/translation_service/transports/base.py
index 204e32ec..5b67ea9a 100644
--- a/google/cloud/translate_v3/services/translation_service/transports/base.py
+++ b/google/cloud/translate_v3/services/translation_service/transports/base.py
@@ -17,9 +17,11 @@
 
 import abc
 import typing
+import packaging.version
 import pkg_resources
 
 from google import auth  # type: ignore
+import google.api_core
 from google.api_core import exceptions  # type: ignore
 from google.api_core import gapic_v1  # type: ignore
 from google.api_core import retry as retries  # type: ignore
@@ -37,6 +39,17 @@
 except pkg_resources.DistributionNotFound:
     DEFAULT_CLIENT_INFO = gapic_v1.client_info.ClientInfo()
 
+try:
+    # google.auth.__version__ was added in 1.26.0
+    _GOOGLE_AUTH_VERSION = auth.__version__
+except AttributeError:
+    try:  # try pkg_resources if it is available
+        _GOOGLE_AUTH_VERSION = pkg_resources.get_distribution("google-auth").version
+    except pkg_resources.DistributionNotFound:  # pragma: NO COVER
+        _GOOGLE_AUTH_VERSION = None
+
+_API_CORE_VERSION = google.api_core.__version__
+
 
 class TranslationServiceTransport(abc.ABC):
     """Abstract transport class for TranslationService."""
@@ -45,14 +58,15 @@ class TranslationServiceTransport(abc.ABC):
         "https://www.googleapis.com/auth/cloud-platform",
         "https://www.googleapis.com/auth/cloud-translation",
     )
+    DEFAULT_HOST = "translate.googleapis.com"
 
     def __init__(
         self,
         *,
-        host: str = "translate.googleapis.com",
+        host: str = DEFAULT_HOST,
         credentials: credentials.Credentials = None,
         credentials_file: typing.Optional[str] = None,
-        scopes: typing.Optional[typing.Sequence[str]] = AUTH_SCOPES,
+        scopes: typing.Optional[typing.Sequence[str]] = None,
         quota_project_id: typing.Optional[str] = None,
         client_info: gapic_v1.client_info.ClientInfo = DEFAULT_CLIENT_INFO,
         **kwargs,
@@ -69,7 +83,7 @@ def __init__(
             credentials_file (Optional[str]): A file with credentials that can
                 be loaded with :func:`google.auth.load_credentials_from_file`.
                 This argument is mutually exclusive with credentials.
-            scope (Optional[Sequence[str]]): A list of scopes.
+            scopes (Optional[Sequence[str]]): A list of scopes.
             quota_project_id (Optional[str]): An optional project to use for billing
                 and quota.
             client_info (google.api_core.gapic_v1.client_info.ClientInfo):	
@@ -83,6 +97,21 @@ def __init__(
             host += ":443"
         self._host = host
 
+        # If a custom API endpoint is set, set scopes to ensure the auth
+        # library does not used the self-signed JWT flow for service
+        # accounts
+        if host.split(":")[0] != self.DEFAULT_HOST and not scopes:
+            scopes = self.AUTH_SCOPES
+
+        # TODO: Remove this if/else once google-auth >= 1.25.0 is required
+        if _GOOGLE_AUTH_VERSION and (
+            packaging.version.parse(_GOOGLE_AUTH_VERSION)
+            >= packaging.version.parse("1.25.0")
+        ):
+            scopes_kwargs = {"scopes": scopes, "default_scopes": self.AUTH_SCOPES}
+        else:
+            scopes_kwargs = {"scopes": scopes or self.AUTH_SCOPES}
+
         # If no credentials are provided, then determine the appropriate
         # defaults.
         if credentials and credentials_file:
@@ -92,12 +121,12 @@ def __init__(
 
         if credentials_file is not None:
             credentials, _ = auth.load_credentials_from_file(
-                credentials_file, scopes=scopes, quota_project_id=quota_project_id
+                credentials_file, **scopes_kwargs, quota_project_id=quota_project_id
             )
 
         elif credentials is None:
             credentials, _ = auth.default(
-                scopes=scopes, quota_project_id=quota_project_id
+                **scopes_kwargs, quota_project_id=quota_project_id
             )
 
         # Save the credentials.
diff --git a/google/cloud/translate_v3/services/translation_service/transports/grpc.py b/google/cloud/translate_v3/services/translation_service/transports/grpc.py
index 7efc5785..142f2fc2 100644
--- a/google/cloud/translate_v3/services/translation_service/transports/grpc.py
+++ b/google/cloud/translate_v3/services/translation_service/transports/grpc.py
@@ -24,6 +24,8 @@
 from google import auth  # type: ignore
 from google.auth import credentials  # type: ignore
 from google.auth.transport.grpc import SslCredentials  # type: ignore
+import packaging
+import pkg_resources
 
 import grpc  # type: ignore
 
@@ -31,6 +33,7 @@
 from google.longrunning import operations_pb2 as operations  # type: ignore
 
 from .base import TranslationServiceTransport, DEFAULT_CLIENT_INFO
+from .base import _GOOGLE_AUTH_VERSION, _API_CORE_VERSION
 
 
 class TranslationServiceGrpcTransport(TranslationServiceTransport):
@@ -105,6 +108,21 @@ def __init__(
         """
         self._ssl_channel_credentials = ssl_channel_credentials
 
+        # If a custom API endpoint is set, set scopes to ensure the auth
+        # library does not used the self-signed JWT flow for service
+        # accounts
+        if host.split(":")[0] != self.DEFAULT_HOST and not scopes:
+            scopes = self.AUTH_SCOPES
+
+        # TODO: Remove this if/else once google-auth >= 1.25.0 is required
+        if _GOOGLE_AUTH_VERSION and (
+            packaging.version.parse(_GOOGLE_AUTH_VERSION)
+            >= packaging.version.parse("1.25.0")
+        ):
+            scopes_kwargs = {"scopes": scopes, "default_scopes": self.AUTH_SCOPES}
+        else:
+            scopes_kwargs = {"scopes": scopes or self.AUTH_SCOPES}
+
         if channel:
             # Sanity check: Ensure that channel and credentials are not both
             # provided.
@@ -127,7 +145,7 @@ def __init__(
 
             if credentials is None:
                 credentials, _ = auth.default(
-                    scopes=self.AUTH_SCOPES, quota_project_id=quota_project_id
+                    **scopes_kwargs, quota_project_id=quota_project_id,
                 )
 
             # Create SSL credentials with client_cert_source or application
@@ -146,7 +164,7 @@ def __init__(
                 credentials=credentials,
                 credentials_file=credentials_file,
                 ssl_credentials=ssl_credentials,
-                scopes=scopes or self.AUTH_SCOPES,
+                scopes=scopes,
                 quota_project_id=quota_project_id,
                 options=[
                     ("grpc.max_send_message_length", -1),
@@ -159,7 +177,7 @@ def __init__(
 
             if credentials is None:
                 credentials, _ = auth.default(
-                    scopes=self.AUTH_SCOPES, quota_project_id=quota_project_id
+                    **scopes_kwargs, quota_project_id=quota_project_id,
                 )
 
             # create a new channel. The provided one is ignored.
@@ -168,7 +186,7 @@ def __init__(
                 credentials=credentials,
                 credentials_file=credentials_file,
                 ssl_credentials=ssl_channel_credentials,
-                scopes=scopes or self.AUTH_SCOPES,
+                scopes=scopes,
                 quota_project_id=quota_project_id,
                 options=[
                     ("grpc.max_send_message_length", -1),
@@ -184,7 +202,7 @@ def __init__(
             host=host,
             credentials=credentials,
             credentials_file=credentials_file,
-            scopes=scopes or self.AUTH_SCOPES,
+            scopes=scopes,
             quota_project_id=quota_project_id,
             client_info=client_info,
         )
@@ -224,13 +242,26 @@ def create_channel(
             google.api_core.exceptions.DuplicateCredentialArgs: If both ``credentials``
               and ``credentials_file`` are passed.
         """
-        scopes = scopes or cls.AUTH_SCOPES
+
+        self_signed_jwt_kwargs = {}
+
+        # TODO: Remove this if/else once google-api-core >= 1.26.0 is required
+        if _API_CORE_VERSION and (
+            packaging.version.parse(_API_CORE_VERSION)
+            >= packaging.version.parse("1.26.0")
+        ):
+            self_signed_jwt_kwargs["default_scopes"] = cls.AUTH_SCOPES
+            self_signed_jwt_kwargs["scopes"] = scopes
+            self_signed_jwt_kwargs["default_host"] = cls.DEFAULT_HOST
+        else:
+            self_signed_jwt_kwargs["scopes"] = scopes or cls.AUTH_SCOPES
+
         return grpc_helpers.create_channel(
             host,
             credentials=credentials,
             credentials_file=credentials_file,
-            scopes=scopes,
             quota_project_id=quota_project_id,
+            **self_signed_jwt_kwargs,
             **kwargs,
         )
 
diff --git a/google/cloud/translate_v3/services/translation_service/transports/grpc_asyncio.py b/google/cloud/translate_v3/services/translation_service/transports/grpc_asyncio.py
index d63d8c0b..586777cf 100644
--- a/google/cloud/translate_v3/services/translation_service/transports/grpc_asyncio.py
+++ b/google/cloud/translate_v3/services/translation_service/transports/grpc_asyncio.py
@@ -24,6 +24,7 @@
 from google import auth  # type: ignore
 from google.auth import credentials  # type: ignore
 from google.auth.transport.grpc import SslCredentials  # type: ignore
+import packaging.version
 
 import grpc  # type: ignore
 from grpc.experimental import aio  # type: ignore
@@ -32,6 +33,7 @@
 from google.longrunning import operations_pb2 as operations  # type: ignore
 
 from .base import TranslationServiceTransport, DEFAULT_CLIENT_INFO
+from .base import _GOOGLE_AUTH_VERSION, _API_CORE_VERSION
 from .grpc import TranslationServiceGrpcTransport
 
 
@@ -82,13 +84,25 @@ def create_channel(
         Returns:
             aio.Channel: A gRPC AsyncIO channel object.
         """
-        scopes = scopes or cls.AUTH_SCOPES
+        self_signed_jwt_kwargs = {}
+
+        # TODO: Remove this if/else once google-api-core >= 1.26.0 is required
+        if _API_CORE_VERSION and (
+            packaging.version.parse(_API_CORE_VERSION)
+            >= packaging.version.parse("1.26.0")
+        ):
+            self_signed_jwt_kwargs["default_scopes"] = cls.AUTH_SCOPES
+            self_signed_jwt_kwargs["scopes"] = scopes
+            self_signed_jwt_kwargs["default_host"] = cls.DEFAULT_HOST
+        else:
+            self_signed_jwt_kwargs["scopes"] = scopes or cls.AUTH_SCOPES
+
         return grpc_helpers_async.create_channel(
             host,
             credentials=credentials,
             credentials_file=credentials_file,
-            scopes=scopes,
             quota_project_id=quota_project_id,
+            **self_signed_jwt_kwargs,
             **kwargs,
         )
 
@@ -150,6 +164,20 @@ def __init__(
         """
         self._ssl_channel_credentials = ssl_channel_credentials
 
+        # If a custom API endpoint is set, set scopes to ensure the auth
+        # library does not used the self-signed JWT flow for service
+        # accounts
+        if host.split(":")[0] != self.DEFAULT_HOST and not scopes:
+            scopes = self.AUTH_SCOPES
+
+        # TODO: Remove this if/else once google-auth >= 1.25.0 is required
+        if _GOOGLE_AUTH_VERSION and packaging.version.parse(
+            _GOOGLE_AUTH_VERSION
+        ) >= packaging.version.parse("1.25.0"):
+            scopes_kwargs = {"scopes": scopes, "default_scopes": self.AUTH_SCOPES}
+        else:
+            scopes_kwargs = {"scopes": scopes or self.AUTH_SCOPES}
+
         if channel:
             # Sanity check: Ensure that channel and credentials are not both
             # provided.
@@ -172,7 +200,7 @@ def __init__(
 
             if credentials is None:
                 credentials, _ = auth.default(
-                    scopes=self.AUTH_SCOPES, quota_project_id=quota_project_id
+                    quota_project_id=quota_project_id, **scopes_kwargs
                 )
 
             # Create SSL credentials with client_cert_source or application
@@ -191,7 +219,7 @@ def __init__(
                 credentials=credentials,
                 credentials_file=credentials_file,
                 ssl_credentials=ssl_credentials,
-                scopes=scopes or self.AUTH_SCOPES,
+                scopes=scopes,
                 quota_project_id=quota_project_id,
                 options=[
                     ("grpc.max_send_message_length", -1),
@@ -204,7 +232,7 @@ def __init__(
 
             if credentials is None:
                 credentials, _ = auth.default(
-                    scopes=self.AUTH_SCOPES, quota_project_id=quota_project_id
+                    quota_project_id=quota_project_id, **scopes_kwargs,
                 )
 
             # create a new channel. The provided one is ignored.
@@ -213,7 +241,7 @@ def __init__(
                 credentials=credentials,
                 credentials_file=credentials_file,
                 ssl_credentials=ssl_channel_credentials,
-                scopes=scopes or self.AUTH_SCOPES,
+                scopes=scopes,
                 quota_project_id=quota_project_id,
                 options=[
                     ("grpc.max_send_message_length", -1),
@@ -226,7 +254,7 @@ def __init__(
             host=host,
             credentials=credentials,
             credentials_file=credentials_file,
-            scopes=scopes or self.AUTH_SCOPES,
+            scopes=scopes,
             quota_project_id=quota_project_id,
             client_info=client_info,
         )
diff --git a/noxfile.py b/noxfile.py
index 8004482e..0e3f1430 100644
--- a/noxfile.py
+++ b/noxfile.py
@@ -18,6 +18,9 @@
 
 from __future__ import absolute_import
 import os
+import sys
+import pathlib
+import subprocess
 import shutil
 
 import nox
@@ -30,6 +33,11 @@
 SYSTEM_TEST_PYTHON_VERSIONS = ["3.8"]
 UNIT_TEST_PYTHON_VERSIONS = ["3.6", "3.7", "3.8", "3.9"]
 
+CURRENT_DIRECTORY = pathlib.Path(__file__).parent.absolute()
+
+LOWER_BOUND_CONSTRAINTS_FILE = CURRENT_DIRECTORY / "testing" / "constraints-3.6.txt"
+PACKAGE_NAME = package_name = subprocess.check_output([sys.executable, "setup.py", "--name"], encoding='utf-8').strip()
+
 
 @nox.session(python=DEFAULT_PYTHON_VERSION)
 def lint(session):
@@ -69,13 +77,17 @@ def lint_setup_py(session):
 
 
 def default(session):
+    constraints_path = str(
+        CURRENT_DIRECTORY / "testing" / f"constraints-{session.python}.txt"
+    )
+
     # Install all test dependencies, then install this package in-place.
     session.install("asyncmock", "pytest-asyncio")
 
     session.install(
         "mock", "pytest", "pytest-cov",
     )
-    session.install("-e", ".")
+    session.install("-e", ".", "-c", constraints_path)
 
     # Run py.test against the unit tests.
     session.run(
@@ -101,6 +113,11 @@ def unit(session):
 @nox.session(python=SYSTEM_TEST_PYTHON_VERSIONS)
 def system(session):
     """Run the system test suite."""
+
+    constraints_path = str(
+        CURRENT_DIRECTORY / "testing" / f"constraints-{session.python}.txt"
+    )
+
     system_test_path = os.path.join("tests", "system.py")
     system_test_folder_path = os.path.join("tests", "system")
 
@@ -125,7 +142,16 @@ def system(session):
     session.install(
         "mock", "pytest", "google-cloud-testutils",
     )
-    session.install("-e", ".")
+    session.install("-e", ".", "-c", constraints_path)
+
+    # Temporarily install google-api-core from HEAD to test self-signed jwt
+    session.install(
+        "-e",
+        "git+https://github.com/googleapis/python-api-core.git@master#egg=google-api-core",
+    )
+
+    # Install google-auth *again* since it will have been overwitten by api-core
+    session.install("google-auth", "-c", constraints_path)
 
     # Run py.test against the system tests.
     if system_test_exists:
@@ -147,6 +173,37 @@ def cover(session):
     session.run("coverage", "erase")
 
 
+@nox.session(python="3.8")
+def check_lower_bounds(session):
+    """Check lower bounds in setup.py are reflected in constraints file"""
+    session.install("google-cloud-testutils")
+    session.install(".")
+
+    session.run(
+        "lower-bound-checker",
+        "check",
+        "--package-name",
+        PACKAGE_NAME,
+        "--constraints-file",
+        str(LOWER_BOUND_CONSTRAINTS_FILE),
+    )
+
+
+@nox.session(python="3.8")
+def update_lower_bounds(session):
+    """Update lower bounds in constraints.txt to match setup.py"""
+    session.install("google-cloud-testutils")
+    session.install(".")
+
+    session.run(
+        "lower-bound-checker",
+        "update",
+        "--package-name",
+        PACKAGE_NAME,
+        "--constraints-file",
+        str(LOWER_BOUND_CONSTRAINTS_FILE),
+    )
+
 @nox.session(python=DEFAULT_PYTHON_VERSION)
 def docs(session):
     """Build the docs for this library."""
diff --git a/setup.py b/setup.py
index 9045fdcf..1bd0bca9 100644
--- a/setup.py
+++ b/setup.py
@@ -29,10 +29,10 @@
 # 'Development Status :: 5 - Production/Stable'
 release_status = "Development Status :: 5 - Production/Stable"
 dependencies = [
-    "google-api-core[grpc] >= 1.22.0, < 2.0.0dev",
-    "google-cloud-core >= 1.1.0, < 2.0dev",
-    "libcst >= 0.2.5",
-    "proto-plus >= 0.4.0",
+    "google-api-core[grpc] >= 1.22.2, < 2.0.0dev",
+    "google-cloud-core >= 1.4.0, < 2.0dev",
+    "proto-plus >= 1.4.0",
+    "packaging >= 14.3",
 ]
 extras = {}
 
diff --git a/testing/constraints-3.10.txt b/testing/constraints-3.10.txt
new file mode 100644
index 00000000..e69de29b
diff --git a/testing/constraints-3.11.txt b/testing/constraints-3.11.txt
new file mode 100644
index 00000000..e69de29b
diff --git a/testing/constraints-3.6.txt b/testing/constraints-3.6.txt
new file mode 100644
index 00000000..81b134a7
--- /dev/null
+++ b/testing/constraints-3.6.txt
@@ -0,0 +1,4 @@
+google-api-core==1.22.2
+google-cloud-core==1.4.0
+packaging==14.3
+proto-plus==1.4.0
\ No newline at end of file
diff --git a/testing/constraints-3.7.txt b/testing/constraints-3.7.txt
new file mode 100644
index 00000000..e69de29b
diff --git a/testing/constraints-3.8.txt b/testing/constraints-3.8.txt
new file mode 100644
index 00000000..e69de29b
diff --git a/testing/constraints-3.9.txt b/testing/constraints-3.9.txt
new file mode 100644
index 00000000..e69de29b
diff --git a/tests/unit/gapic/translate_v3/test_translation_service.py b/tests/unit/gapic/translate_v3/test_translation_service.py
index 215b08b1..3a1f061a 100644
--- a/tests/unit/gapic/translate_v3/test_translation_service.py
+++ b/tests/unit/gapic/translate_v3/test_translation_service.py
@@ -21,6 +21,7 @@
 import grpc
 from grpc.experimental import aio
 import math
+import packaging.version
 import pytest
 from proto.marshal.rules.dates import DurationRule, TimestampRule
 
@@ -43,12 +44,41 @@
 )
 from google.cloud.translate_v3.services.translation_service import pagers
 from google.cloud.translate_v3.services.translation_service import transports
+from google.cloud.translate_v3.services.translation_service.transports.base import (
+    _GOOGLE_AUTH_VERSION,
+)
+from google.cloud.translate_v3.services.translation_service.transports.base import (
+    _API_CORE_VERSION,
+)
 from google.cloud.translate_v3.types import translation_service
 from google.longrunning import operations_pb2
 from google.oauth2 import service_account
 from google.protobuf import timestamp_pb2 as timestamp  # type: ignore
 
 
+# TODO(busunkim): Once google-api-core >= 1.26.0 is required:
+# - Delete all the api-core and auth "less than" test cases
+# - Delete these pytest markers (Make the "greater than or equal to" tests the default).
+requires_google_auth_lt_1_25_0 = pytest.mark.skipif(
+    packaging.version.parse(_GOOGLE_AUTH_VERSION) >= packaging.version.parse("1.25.0"),
+    reason="This test requires google-auth < 1.25.0",
+)
+requires_google_auth_gte_1_25_0 = pytest.mark.skipif(
+    packaging.version.parse(_GOOGLE_AUTH_VERSION) < packaging.version.parse("1.25.0"),
+    reason="This test requires google-auth >= 1.25.0",
+)
+
+requires_api_core_lt_1_26_0 = pytest.mark.skipif(
+    packaging.version.parse(_API_CORE_VERSION) >= packaging.version.parse("1.26.0"),
+    reason="This test requires google-api-core < 1.26.0",
+)
+
+requires_api_core_gte_1_26_0 = pytest.mark.skipif(
+    packaging.version.parse(_API_CORE_VERSION) < packaging.version.parse("1.26.0"),
+    reason="This test requires google-api-core >= 1.26.0",
+)
+
+
 def client_cert_source_callback():
     return b"cert bytes", b"key bytes"
 
@@ -2367,6 +2397,7 @@ def test_translation_service_base_transport():
         transport.operations_client
 
 
+@requires_google_auth_gte_1_25_0
 def test_translation_service_base_transport_with_credentials_file():
     # Instantiate the base transport with a credentials file
     with mock.patch.object(
@@ -2381,17 +2412,71 @@ def test_translation_service_base_transport_with_credentials_file():
         )
         load_creds.assert_called_once_with(
             "credentials.json",
+            scopes=None,
+            default_scopes=(
+                "https://www.googleapis.com/auth/cloud-platform",
+                "https://www.googleapis.com/auth/cloud-translation",
+            ),
+            quota_project_id="octopus",
+        )
+
+
+@requires_google_auth_lt_1_25_0
+def test_translation_service_base_transport_with_credentials_file_old_google_auth():
+    # Instantiate the base transport with a credentials file
+    with mock.patch.object(
+        auth, "load_credentials_from_file"
+    ) as load_creds, mock.patch(
+        "google.cloud.translate_v3.services.translation_service.transports.TranslationServiceTransport._prep_wrapped_messages"
+    ) as Transport:
+        Transport.return_value = None
+        load_creds.return_value = (credentials.AnonymousCredentials(), None)
+        transport = transports.TranslationServiceTransport(
+            credentials_file="credentials.json", quota_project_id="octopus",
+        )
+        load_creds.assert_called_once_with(
+            "credentials.json",
+            scopes=(
+                "https://www.googleapis.com/auth/cloud-platform",
+                "https://www.googleapis.com/auth/cloud-translation",
+            ),
+            quota_project_id="octopus",
+        )
+
+
+@requires_google_auth_gte_1_25_0
+def test_translation_service_base_transport_custom_host():
+    # Instantiate the base transport with a credentials file
+    with mock.patch.object(
+        auth, "load_credentials_from_file"
+    ) as load_creds, mock.patch(
+        "google.cloud.translate_v3.services.translation_service.transports.TranslationServiceTransport._prep_wrapped_messages"
+    ) as Transport:
+        Transport.return_value = None
+        load_creds.return_value = (credentials.AnonymousCredentials(), None)
+        transport = transports.TranslationServiceTransport(
+            host="squid.clam.whelk",
+            credentials_file="credentials.json",
+            quota_project_id="octopus",
+        )
+        load_creds.assert_called_once_with(
+            "credentials.json",
+            # scopes should be set since custom endpoint was passed
             scopes=(
                 "https://www.googleapis.com/auth/cloud-platform",
                 "https://www.googleapis.com/auth/cloud-translation",
             ),
+            default_scopes=(
+                "https://www.googleapis.com/auth/cloud-platform",
+                "https://www.googleapis.com/auth/cloud-translation",
+            ),
             quota_project_id="octopus",
         )
 
 
 def test_translation_service_base_transport_with_adc():
     # Test the default credentials are used if credentials and credentials_file are None.
-    with mock.patch.object(auth, "default") as adc, mock.patch(
+    with mock.patch.object(auth, "default", autospec=True) as adc, mock.patch(
         "google.cloud.translate_v3.services.translation_service.transports.TranslationServiceTransport._prep_wrapped_messages"
     ) as Transport:
         Transport.return_value = None
@@ -2400,9 +2485,26 @@ def test_translation_service_base_transport_with_adc():
         adc.assert_called_once()
 
 
+@requires_google_auth_gte_1_25_0
 def test_translation_service_auth_adc():
     # If no credentials are provided, we should use ADC credentials.
-    with mock.patch.object(auth, "default") as adc:
+    with mock.patch.object(auth, "default", autospec=True) as adc:
+        adc.return_value = (credentials.AnonymousCredentials(), None)
+        TranslationServiceClient()
+        adc.assert_called_once_with(
+            scopes=None,
+            default_scopes=(
+                "https://www.googleapis.com/auth/cloud-platform",
+                "https://www.googleapis.com/auth/cloud-translation",
+            ),
+            quota_project_id=None,
+        )
+
+
+@requires_google_auth_lt_1_25_0
+def test_translation_service_auth_adc_old_google_auth():
+    # If no credentials are provided, we should use ADC credentials.
+    with mock.patch.object(auth, "default", autospec=True) as adc:
         adc.return_value = (credentials.AnonymousCredentials(), None)
         TranslationServiceClient()
         adc.assert_called_once_with(
@@ -2414,14 +2516,72 @@ def test_translation_service_auth_adc():
         )
 
 
-def test_translation_service_transport_auth_adc():
+@pytest.mark.parametrize(
+    "transport_class",
+    [
+        transports.TranslationServiceGrpcTransport,
+        transports.TranslationServiceGrpcAsyncIOTransport,
+    ],
+)
+@requires_google_auth_gte_1_25_0
+def test_translation_service_transport_auth_adc(transport_class):
+    # If credentials and host are not provided, the transport class should use
+    # ADC credentials.
+    with mock.patch.object(auth, "default", autospec=True) as adc:
+        adc.return_value = (credentials.AnonymousCredentials(), None)
+        transport_class(quota_project_id="octopus", scopes=["1", "2"])
+        adc.assert_called_once_with(
+            scopes=["1", "2"],
+            default_scopes=(
+                "https://www.googleapis.com/auth/cloud-platform",
+                "https://www.googleapis.com/auth/cloud-translation",
+            ),
+            quota_project_id="octopus",
+        )
+
+
+@pytest.mark.parametrize(
+    "transport_class",
+    [
+        transports.TranslationServiceGrpcTransport,
+        transports.TranslationServiceGrpcAsyncIOTransport,
+    ],
+)
+@requires_google_auth_gte_1_25_0
+def test_translation_service_transport_auth_adc_custom_host(transport_class):
     # If credentials and host are not provided, the transport class should use
     # ADC credentials.
     with mock.patch.object(auth, "default") as adc:
         adc.return_value = (credentials.AnonymousCredentials(), None)
-        transports.TranslationServiceGrpcTransport(
-            host="squid.clam.whelk", quota_project_id="octopus"
+        transport_class(host="squid.clam.whelk", quota_project_id="octopus")
+        adc.assert_called_once_with(
+            default_scopes=(
+                "https://www.googleapis.com/auth/cloud-platform",
+                "https://www.googleapis.com/auth/cloud-translation",
+            ),
+            # scopes should be set since a custom endpoint (host) was set
+            scopes=(
+                "https://www.googleapis.com/auth/cloud-platform",
+                "https://www.googleapis.com/auth/cloud-translation",
+            ),
+            quota_project_id="octopus",
         )
+
+
+@pytest.mark.parametrize(
+    "transport_class",
+    [
+        transports.TranslationServiceGrpcTransport,
+        transports.TranslationServiceGrpcAsyncIOTransport,
+    ],
+)
+@requires_google_auth_lt_1_25_0
+def test_translation_service_transport_auth_adc_old_google_auth(transport_class):
+    # If credentials and host are not provided, the transport class should use
+    # ADC credentials.
+    with mock.patch.object(auth, "default", autospec=True) as adc:
+        adc.return_value = (credentials.AnonymousCredentials(), None)
+        transport_class(quota_project_id="octopus")
         adc.assert_called_once_with(
             scopes=(
                 "https://www.googleapis.com/auth/cloud-platform",
@@ -2431,6 +2591,112 @@ def test_translation_service_transport_auth_adc():
         )
 
 
+@pytest.mark.parametrize(
+    "transport_class,grpc_helpers",
+    [
+        (transports.TranslationServiceGrpcTransport, grpc_helpers),
+        (transports.TranslationServiceGrpcAsyncIOTransport, grpc_helpers_async),
+    ],
+)
+@requires_api_core_gte_1_26_0
+def test_translation_service_transport_create_channel(transport_class, grpc_helpers):
+    # If credentials and host are not provided, the transport class should use
+    # ADC credentials.
+    with mock.patch.object(auth, "default", autospec=True) as adc, mock.patch.object(
+        grpc_helpers, "create_channel", autospec=True
+    ) as create_channel:
+        creds = credentials.AnonymousCredentials()
+        adc.return_value = (creds, None)
+        transport_class(quota_project_id="octopus", scopes=["1", "2"])
+
+        create_channel.assert_called_with(
+            "translate.googleapis.com:443",
+            credentials=creds,
+            credentials_file=None,
+            quota_project_id="octopus",
+            default_scopes=(
+                "https://www.googleapis.com/auth/cloud-platform",
+                "https://www.googleapis.com/auth/cloud-translation",
+            ),
+            scopes=["1", "2"],
+            default_host="translate.googleapis.com",
+            ssl_credentials=None,
+            options=[
+                ("grpc.max_send_message_length", -1),
+                ("grpc.max_receive_message_length", -1),
+            ],
+        )
+
+
+@pytest.mark.parametrize(
+    "transport_class,grpc_helpers",
+    [
+        (transports.TranslationServiceGrpcTransport, grpc_helpers),
+        (transports.TranslationServiceGrpcAsyncIOTransport, grpc_helpers_async),
+    ],
+)
+@requires_api_core_lt_1_26_0
+def test_translation_service_transport_create_channel(transport_class, grpc_helpers):
+    # If credentials and host are not provided, the transport class should use
+    # ADC credentials.
+    with mock.patch.object(auth, "default", autospec=True) as adc, mock.patch.object(
+        grpc_helpers, "create_channel", autospec=True
+    ) as create_channel:
+        creds = credentials.AnonymousCredentials()
+        adc.return_value = (creds, None)
+        transport_class(quota_project_id="octopus")
+
+        create_channel.assert_called_with(
+            "translate.googleapis.com:443",
+            credentials=creds,
+            credentials_file=None,
+            quota_project_id="octopus",
+            scopes=(
+                "https://www.googleapis.com/auth/cloud-platform",
+                "https://www.googleapis.com/auth/cloud-translation",
+            ),
+            ssl_credentials=None,
+            options=[
+                ("grpc.max_send_message_length", -1),
+                ("grpc.max_receive_message_length", -1),
+            ],
+        )
+
+
+@pytest.mark.parametrize(
+    "transport_class,grpc_helpers",
+    [
+        (transports.TranslationServiceGrpcTransport, grpc_helpers),
+        (transports.TranslationServiceGrpcAsyncIOTransport, grpc_helpers_async),
+    ],
+)
+@requires_api_core_lt_1_26_0
+def test_translation_service_transport_create_channel_old_api_core_user_scopes(
+    transport_class, grpc_helpers
+):
+    # If credentials and host are not provided, the transport class should use
+    # ADC credentials.
+    with mock.patch.object(auth, "default", autospec=True) as adc, mock.patch.object(
+        grpc_helpers, "create_channel", autospec=True
+    ) as create_channel:
+        creds = credentials.AnonymousCredentials()
+        adc.return_value = (creds, None)
+        transport_class(quota_project_id="octopus", scopes=["1", "2"])
+
+        create_channel.assert_called_with(
+            "translate.googleapis.com:443",
+            credentials=creds,
+            credentials_file=None,
+            quota_project_id="octopus",
+            scopes=["1", "2"],
+            ssl_credentials=None,
+            options=[
+                ("grpc.max_send_message_length", -1),
+                ("grpc.max_receive_message_length", -1),
+            ],
+        )
+
+
 def test_translation_service_host_no_port():
     client = TranslationServiceClient(
         credentials=credentials.AnonymousCredentials(),