diff --git a/lib/meta.dart b/lib/meta.dart
index 8547ffb32..8d16e62a8 100644
--- a/lib/meta.dart
+++ b/lib/meta.dart
@@ -48,6 +48,7 @@ const List<String> GeneratedMessage_reservedNames = const [
   'eventPlugin',
   'createRepeatedField',
   'unknownFields',
+  'clone',
   r'$_get',
   r'$_has',
   r'$_setBool',
diff --git a/lib/src/protobuf/generated_message.dart b/lib/src/protobuf/generated_message.dart
index 8cfd5f66f..ce254d115 100644
--- a/lib/src/protobuf/generated_message.dart
+++ b/lib/src/protobuf/generated_message.dart
@@ -44,6 +44,10 @@ abstract class GeneratedMessage {
   /// to protobuf fields.
   EventPlugin get eventPlugin => null;
 
+  /// Creates a deep copy of the fields in this message.
+  /// (The generated code uses [mergeFromMessage].)
+  GeneratedMessage clone();
+
   UnknownFieldSet get unknownFields => _fieldSet._ensureUnknownFields();
 
   bool get _isReadOnly => false;
diff --git a/test/readonly_message_test.dart b/test/readonly_message_test.dart
index c6f11f4eb..5cbad018f 100644
--- a/test/readonly_message_test.dart
+++ b/test/readonly_message_test.dart
@@ -20,6 +20,8 @@ throwsError(Type expectedType, String expectedMessage) => throwsA(
 class Rec extends GeneratedMessage with ReadonlyMessageMixin {
   @override
   BuilderInfo info_ = new BuilderInfo("rec");
+  @override
+  clone() => throw new UnimplementedError();
 }
 
 main() {