diff --git a/Project.toml b/Project.toml
index 7bfa4da6..8ef5035d 100644
--- a/Project.toml
+++ b/Project.toml
@@ -8,6 +8,7 @@ ArgCheck = "dce04be8-c92d-5529-be00-80e4d2c0e197"
 BangBang = "198e06fe-97b7-11e9-32a5-e1d131e6ad66"
 ConstructionBase = "187b0558-2788-49d3-abe0-74a17ed4e7c9"
 InitialValues = "22cec73e-a1b8-11e9-2c92-598750a2cf9c"
+MicroCollections = "128add7d-3638-4c79-886c-908ea0c25c34"
 Referenceables = "42d2dcc6-99eb-4e98-b66c-637b7d73030e"
 Setfield = "efcf1570-3423-57d1-acb7-fd33fddbac46"
 SplittablesBase = "171d559e-b47b-412a-8079-5efa626c420e"
@@ -18,10 +19,11 @@ ArgCheck = "1.0, 2.0"
 BangBang = "0.3.17"
 ConstructionBase = "0.1.0, 1.0"
 InitialValues = "0.2"
+MicroCollections = "0.1"
 Referenceables = "0.1"
 Setfield = "0.3, 0.4, 0.5, 0.6, 0.7"
 SplittablesBase = "0.1.8"
-Transducers = "0.4.39"
+Transducers = "0.4.52"
 julia = "1"
 
 [extras]
diff --git a/benchmark/Manifest.toml b/benchmark/Manifest.toml
index 32d49a72..7f2cab27 100644
--- a/benchmark/Manifest.toml
+++ b/benchmark/Manifest.toml
@@ -126,6 +126,12 @@ version = "0.5.5"
 deps = ["Base64"]
 uuid = "d6f4376e-aef5-505a-96c1-9c027394607a"
 
+[[MicroCollections]]
+deps = ["BangBang", "Setfield"]
+git-tree-sha1 = "e991b6a9d38091c4a0d7cd051fcb57c05f98ac03"
+uuid = "128add7d-3638-4c79-886c-908ea0c25c34"
+version = "0.1.0"
+
 [[Mmap]]
 uuid = "a63ad114-7e13-5084-954f-fe012c677804"
 
@@ -232,18 +238,18 @@ deps = ["Distributed", "InteractiveUtils", "Logging", "Random"]
 uuid = "8dfed614-e22c-5e08-85e1-65c5234f0b40"
 
 [[ThreadsX]]
-deps = ["ArgCheck", "BangBang", "ConstructionBase", "InitialValues", "Referenceables", "Setfield", "SplittablesBase", "Transducers"]
+deps = ["ArgCheck", "BangBang", "ConstructionBase", "InitialValues", "MicroCollections", "Referenceables", "Setfield", "SplittablesBase", "Transducers"]
 path = ".."
 uuid = "ac1d9e8a-700a-412c-b207-f0111f4b6c0d"
 version = "0.1.3-DEV"
 
 [[Transducers]]
-deps = ["ArgCheck", "BangBang", "CompositionsBase", "DefineSingletons", "Distributed", "InitialValues", "Logging", "Markdown", "Requires", "Setfield", "SplittablesBase", "Tables"]
-git-tree-sha1 = "d9bfa17064d3ea0da8213aa315d55cb8c84b60db"
+deps = ["ArgCheck", "BangBang", "CompositionsBase", "DefineSingletons", "Distributed", "InitialValues", "Logging", "Markdown", "MicroCollections", "Requires", "Setfield", "SplittablesBase", "Tables"]
+git-tree-sha1 = "a2d46274b3a58d9117681d7effdf706022d44650"
 repo-rev = "master"
 repo-url = "https://github.com/JuliaFolds/Transducers.jl.git"
 uuid = "28d57a85-8fef-5791-bfe6-a80928e7c999"
-version = "0.4.41-DEV"
+version = "0.4.52"
 
 [[UUIDs]]
 deps = ["Random", "SHA"]
diff --git a/benchmark/bench_mapi.jl b/benchmark/bench_mapi.jl
new file mode 100644
index 00000000..cc010296
--- /dev/null
+++ b/benchmark/bench_mapi.jl
@@ -0,0 +1,52 @@
+module BenchMapi
+
+using BenchmarkTools
+using ThreadsX
+
+collatz(x) =
+    if iseven(x)
+        x ÷ 2
+    else
+        3x + 1
+    end
+
+function collatz_stopping_time(x)
+    n = 0
+    while true
+        x == 1 && return n
+        n += 1
+        x = collatz(x)
+    end
+end
+
+function consume(ns)
+    t0 = time_ns()
+    d = 0
+    while d < ns
+        d = Int(time_ns() - t0)
+    end
+    return d
+end
+
+constant(x) = _ -> x
+
+const SUITE = BenchmarkGroup()
+
+let s0 = SUITE["collatz"] = BenchmarkGroup()
+    s0["base"] = @benchmarkable map(collatz_stopping_time, 1:1000_000)
+    s0["tx"] =
+        @benchmarkable ThreadsX.mapi(collatz_stopping_time, 1:1000_000; basesize = 12500)
+end
+
+let s0 = SUITE["consume-1ms"] = BenchmarkGroup()
+    s0["base"] = @benchmarkable map(consume ∘ constant(1_000_000), 1:20)
+    s0["tx"] = @benchmarkable ThreadsX.mapi(consume ∘ constant(1_000_000), 1:20)
+end
+
+let s0 = SUITE["consume-100us"] = BenchmarkGroup()
+    s0["base"] = @benchmarkable map(consume ∘ constant(100_000), 1:20)
+    s0["tx"] = @benchmarkable ThreadsX.mapi(consume ∘ constant(100_000), 1:20)
+end
+
+end  # module
+BenchMapi.SUITE
diff --git a/docs/Manifest.toml b/docs/Manifest.toml
index 5a226d59..e00222a2 100644
--- a/docs/Manifest.toml
+++ b/docs/Manifest.toml
@@ -132,6 +132,12 @@ version = "0.5.5"
 deps = ["Base64"]
 uuid = "d6f4376e-aef5-505a-96c1-9c027394607a"
 
+[[MicroCollections]]
+deps = ["BangBang", "Setfield"]
+git-tree-sha1 = "e991b6a9d38091c4a0d7cd051fcb57c05f98ac03"
+uuid = "128add7d-3638-4c79-886c-908ea0c25c34"
+version = "0.1.0"
+
 [[Missings]]
 deps = ["DataAPI"]
 git-tree-sha1 = "de0a5ce9e5289f27df672ffabef4d1e5861247d5"
@@ -266,18 +272,18 @@ deps = ["Distributed", "InteractiveUtils", "Logging", "Random"]
 uuid = "8dfed614-e22c-5e08-85e1-65c5234f0b40"
 
 [[ThreadsX]]
-deps = ["ArgCheck", "BangBang", "ConstructionBase", "InitialValues", "Referenceables", "Setfield", "SplittablesBase", "Transducers"]
+deps = ["ArgCheck", "BangBang", "ConstructionBase", "InitialValues", "MicroCollections", "Referenceables", "Setfield", "SplittablesBase", "Transducers"]
 path = ".."
 uuid = "ac1d9e8a-700a-412c-b207-f0111f4b6c0d"
 version = "0.1.3-DEV"
 
 [[Transducers]]
-deps = ["ArgCheck", "BangBang", "CompositionsBase", "DefineSingletons", "Distributed", "InitialValues", "Logging", "Markdown", "Requires", "Setfield", "SplittablesBase", "Tables"]
-git-tree-sha1 = "d9bfa17064d3ea0da8213aa315d55cb8c84b60db"
+deps = ["ArgCheck", "BangBang", "CompositionsBase", "DefineSingletons", "Distributed", "InitialValues", "Logging", "Markdown", "MicroCollections", "Requires", "Setfield", "SplittablesBase", "Tables"]
+git-tree-sha1 = "a2d46274b3a58d9117681d7effdf706022d44650"
 repo-rev = "master"
 repo-url = "https://github.com/JuliaFolds/Transducers.jl.git"
 uuid = "28d57a85-8fef-5791-bfe6-a80928e7c999"
-version = "0.4.41-DEV"
+version = "0.4.52"
 
 [[UUIDs]]
 deps = ["Random", "SHA"]
diff --git a/docs/src/index.md b/docs/src/index.md
index c19836b8..a8bce5b3 100644
--- a/docs/src/index.md
+++ b/docs/src/index.md
@@ -3,6 +3,8 @@
 ```@docs
 ThreadsX
 ThreadsX.foreach
+ThreadsX.map
+ThreadsX.mapi
 ThreadsX.map!
 ThreadsX.sort!
 ThreadsX.sort
diff --git a/src/ThreadsX.jl b/src/ThreadsX.jl
index 1ea6a2d0..3ca438d7 100644
--- a/src/ThreadsX.jl
+++ b/src/ThreadsX.jl
@@ -2,6 +2,7 @@ baremodule ThreadsX
 
 function collect end
 function map end
+function mapi end
 
 function mapreduce end
 function reduce end
@@ -42,6 +43,7 @@ using Base:
     HasShape, IteratorSize, Ordering, add_sum, mapreduce_empty, mul_prod, reduce_empty
 using ConstructionBase: setproperties
 using InitialValues: asmonoid
+using MicroCollections: EmptyVector
 using Referenceables: referenceable
 using Setfield: @set
 using Transducers:
@@ -51,10 +53,12 @@ using Transducers:
     Init,
     Map,
     MapSplat,
+    NondeterministicThreading,
     OnInit,
     ReduceIf,
     Transducers,
     extract_transducer,
+    foldxl,
     opcompose,
     reduced,
     right,
diff --git a/src/docs/map.md b/src/docs/map.md
new file mode 100644
index 00000000..46516ad2
--- /dev/null
+++ b/src/docs/map.md
@@ -0,0 +1,4 @@
+    ThreadsX.mapi(f, iterators...; basesize)
+
+Parallelized `map(f, iterators...)`.  Input collections `iterators`
+must support `SplittablesBase.halve`
diff --git a/src/docs/mapi.md b/src/docs/mapi.md
new file mode 100644
index 00000000..859db4c2
--- /dev/null
+++ b/src/docs/mapi.md
@@ -0,0 +1,21 @@
+    ThreadsX.mapi(f, iterators...; basesize, ntasks)
+
+Parallelized `map(f, iterators...)` that works with purely sequential
+`iterators`.
+
+Note that calls to `iterate` on `iterators` are *not* parallelized.
+Only `f` may be called in parallel.  See also
+`Transducers.NondeterministicThreading` for more information.
+
+!!! note
+    Currently, the default `basesize` is 1.  However, it may be
+    changed in the future (e.g. it may be automatically tuned at
+    run-time).
+
+# Keyword Arguments
+- `basesize::Integer`: The number of input elements to be accumulated
+  in a buffer before sent to a task.
+- `ntasks::Integer`: The number of tasks `@spawn`ed.  The default
+  value is `Threads.nthreads()`.  A number larger than
+  `Threads.nthreads()` may be useful if the inner reducing function
+  contains I/O and does not consume too much resource (e.g., memory).
diff --git a/src/map.jl b/src/map.jl
index 27b9b98a..0958d402 100644
--- a/src/map.jl
+++ b/src/map.jl
@@ -49,3 +49,16 @@ ThreadsX.collect(::Type{T}, itr; kwargs...) where {T} = reshape_as(
 
 ThreadsX.collect(itr; kwargs...) =
     reshape_as(tcollect(itr; basesize = default_basesize(itr), kwargs...), itr)
+
+
+ThreadsX.mapi(f, itr; kwargs...) = reshape_as(
+    itr |>
+    NondeterministicThreading(; kwargs...) |>
+    Map(f) |>
+    Map(SingletonVector) |>
+    foldxl(append!!; init = EmptyVector()),
+    itr,
+)
+
+ThreadsX.mapi(f, itr1, itrs...; kwargs...) =
+    ThreadsX.mapi(Base.splat(f), zip(itr1, itrs...); kwargs...)
diff --git a/test/environments/main/Manifest.toml b/test/environments/main/Manifest.toml
index 2272c941..03fa8a2b 100644
--- a/test/environments/main/Manifest.toml
+++ b/test/environments/main/Manifest.toml
@@ -138,6 +138,12 @@ version = "0.5.5"
 deps = ["Base64"]
 uuid = "d6f4376e-aef5-505a-96c1-9c027394607a"
 
+[[MicroCollections]]
+deps = ["BangBang", "Setfield"]
+git-tree-sha1 = "e991b6a9d38091c4a0d7cd051fcb57c05f98ac03"
+uuid = "128add7d-3638-4c79-886c-908ea0c25c34"
+version = "0.1.0"
+
 [[Missings]]
 deps = ["DataAPI"]
 git-tree-sha1 = "de0a5ce9e5289f27df672ffabef4d1e5861247d5"
@@ -272,18 +278,18 @@ deps = ["Distributed", "InteractiveUtils", "Logging", "Random"]
 uuid = "8dfed614-e22c-5e08-85e1-65c5234f0b40"
 
 [[ThreadsX]]
-deps = ["ArgCheck", "BangBang", "ConstructionBase", "InitialValues", "Referenceables", "Setfield", "SplittablesBase", "Transducers"]
+deps = ["ArgCheck", "BangBang", "ConstructionBase", "InitialValues", "MicroCollections", "Referenceables", "Setfield", "SplittablesBase", "Transducers"]
 path = "../../.."
 uuid = "ac1d9e8a-700a-412c-b207-f0111f4b6c0d"
 version = "0.1.3-DEV"
 
 [[Transducers]]
-deps = ["ArgCheck", "BangBang", "CompositionsBase", "DefineSingletons", "Distributed", "InitialValues", "Logging", "Markdown", "Requires", "Setfield", "SplittablesBase", "Tables"]
-git-tree-sha1 = "d9bfa17064d3ea0da8213aa315d55cb8c84b60db"
+deps = ["ArgCheck", "BangBang", "CompositionsBase", "DefineSingletons", "Distributed", "InitialValues", "Logging", "Markdown", "MicroCollections", "Requires", "Setfield", "SplittablesBase", "Tables"]
+git-tree-sha1 = "a2d46274b3a58d9117681d7effdf706022d44650"
 repo-rev = "master"
 repo-url = "https://github.com/JuliaFolds/Transducers.jl.git"
 uuid = "28d57a85-8fef-5791-bfe6-a80928e7c999"
-version = "0.4.41-DEV"
+version = "0.4.52"
 
 [[UUIDs]]
 deps = ["Random", "SHA"]
diff --git a/test/test_with_base.jl b/test/test_with_base.jl
index d135dbb5..a5a6ad06 100644
--- a/test/test_with_base.jl
+++ b/test/test_with_base.jl
@@ -8,6 +8,8 @@ args_and_kwargs(args...; kwargs...) = args, (; kwargs...)
 
 inc(x) = x + 1
 
+mapi(args...) = map(args...)
+
 raw_testdata = """
 collect(1:10)
 collect(Float64, 1:10)
@@ -103,12 +105,17 @@ Set(x^2 for x in [1, -1, -3, 4, 3])
 """
 
 # An array of `(label, (f, args, kwargs))`
-testdata = map(split(raw_testdata, "\n", keepempty = false)) do x
+testdata = mapreduce(vcat, split(raw_testdata, "\n", keepempty = false)) do x
     @debug "Parsing: $x"
     f, rest = split(x, "(", limit = 2)
     ex = Meta.parse("DUMMY($rest")
     ex.args[1] = args_and_kwargs
-    @eval ($x, ($(Symbol(f)), $ex...))
+    tests = Any[@eval ($x, ($(Symbol(f)), $ex...))]
+    if f == "map"
+        y = replace(x, r"^map" => "mapi")
+        push!(tests, @eval ($y, (mapi, $ex...)))
+    end
+    return tests
 end
 
 @testset "$label" for (label, (f, args, kwargs)) in testdata