diff --git a/polygerrit-ui/app/elements/diff/gr-syntax-layer/gr-syntax-layer.js b/polygerrit-ui/app/elements/diff/gr-syntax-layer/gr-syntax-layer.js
index 1a0bbd9f50..d8d2fa643b 100644
--- a/polygerrit-ui/app/elements/diff/gr-syntax-layer/gr-syntax-layer.js
+++ b/polygerrit-ui/app/elements/diff/gr-syntax-layer/gr-syntax-layer.js
@@ -102,6 +102,10 @@ const LANGUAGE_MAP = {
 };
 const ASYNC_DELAY = 10;
 
+const FILENAME_OVERRIDES = {
+  'rules.pl': 'prolog',
+};
+
 const CLASS_WHITELIST = {
   'gr-diff gr-syntax gr-syntax-attr': true,
   'gr-diff gr-syntax gr-syntax-attribute': true,
@@ -233,10 +237,16 @@ class GrSyntaxLayer extends GestureEventListeners(
     }
   }
 
+  _basename(filename) {
+    const pieces = filename.split(/\//);
+    return pieces[pieces.length-1];
+  }
+
   _getLanguage(diffFileMetaInfo) {
     // The Gerrit API provides only content-type, but for other users of
     // gr-diff it may be more convenient to specify the language directly.
     return diffFileMetaInfo.language ||
+        FILENAME_OVERRIDES[this._basename(diffFileMetaInfo.name)] ||
         LANGUAGE_MAP[diffFileMetaInfo.content_type];
   }