diff --git a/gtfs-realtime/proto/gtfs-realtime.proto b/gtfs-realtime/proto/gtfs-realtime.proto
index 76057b9d4..78a0d038d 100644
--- a/gtfs-realtime/proto/gtfs-realtime.proto
+++ b/gtfs-realtime/proto/gtfs-realtime.proto
@@ -51,7 +51,7 @@ message FeedMessage {
 // Metadata about a feed, included in feed messages.
 message FeedHeader {
   // Version of the feed specification.
-  // The current version is 2.0.
+  // The current version is 2.0.  Valid versions are "2.0", "1.0".
   required string gtfs_realtime_version = 1;
 
   // Determines whether the current fetch is incremental.  Currently,
diff --git a/gtfs-realtime/spec/en/examples/alerts.asciipb b/gtfs-realtime/spec/en/examples/alerts.asciipb
index 601bfd061..b30316e54 100644
--- a/gtfs-realtime/spec/en/examples/alerts.asciipb
+++ b/gtfs-realtime/spec/en/examples/alerts.asciipb
@@ -1,6 +1,6 @@
 # header information
 header {
-  # version of speed specification. Currently "2.0"
+  # version of speed specification. Currently "2.0". Valid versions are "2.0", "1.0".
   gtfs_realtime_version: "2.0"
 
   # determines whether dataset is incremental or full
diff --git a/gtfs-realtime/spec/en/examples/trip-updates-full.asciipb b/gtfs-realtime/spec/en/examples/trip-updates-full.asciipb
index 3dd58a988..fe0755c68 100644
--- a/gtfs-realtime/spec/en/examples/trip-updates-full.asciipb
+++ b/gtfs-realtime/spec/en/examples/trip-updates-full.asciipb
@@ -1,6 +1,6 @@
 # header information
 header {
-  # version of speed specification. Currently "2.0"
+  # version of speed specification. Currently "2.0". Valid versions are "2.0", "1.0".
   gtfs_realtime_version: "2.0"
   # determines whether dataset is incremental or full
   incrementality: FULL_DATASET
diff --git a/gtfs-realtime/spec/en/reference.md b/gtfs-realtime/spec/en/reference.md
index a4a918809..16c377f40 100644
--- a/gtfs-realtime/spec/en/reference.md
+++ b/gtfs-realtime/spec/en/reference.md
@@ -1,6 +1,6 @@
 A GTFS Realtime feed lets transit agencies provide consumers with realtime information about disruptions to their service (stations closed, lines not operating, important delays, etc.) location of their vehicles, and expected arrival times.
 
-Version 2.0 of the feed specification is discussed and documented on this site.
+Version 2.0 of the feed specification is discussed and documented on this site. Valid versions are "2.0", "1.0".
 
 ### Term Definitions