diff --git a/lisp/progmodes/prolog.el b/lisp/progmodes/prolog.el
index 73450dec61a..b6c4dd8c662 100644
--- a/lisp/progmodes/prolog.el
+++ b/lisp/progmodes/prolog.el
@@ -31,7 +31,7 @@
 ;;; Code:
 
 (defgroup prolog nil
-  "Major mode for editing and running Prolog under Emacs"
+  "Major mode for editing and running Prolog under Emacs."
   :group 'languages)