diff --git a/google/cloud/speech/v1/cloud_speech.proto b/google/cloud/speech/v1/cloud_speech.proto
index 9553510a..f343fa21 100644
--- a/google/cloud/speech/v1/cloud_speech.proto
+++ b/google/cloud/speech/v1/cloud_speech.proto
@@ -366,6 +366,10 @@ message SpeakerDiarizationConfig {
   // flexibility by allowing the system to automatically determine the correct
   // number of speakers. If not set, the default value is 6.
   int32 max_speaker_count = 3;
+
+  // Unused.
+  int32 speaker_tag = 5
+      [(google.api.field_behavior) = OUTPUT_ONLY, deprecated = true];
 }
 
 // Description of audio data to be recognized.