diff --git a/lisp/progmodes/prolog.el b/lisp/progmodes/prolog.el
index c8f6c12a3f..742274a479 100644
--- a/lisp/progmodes/prolog.el
+++ b/lisp/progmodes/prolog.el
@@ -1201,7 +1201,8 @@ mercury-mode-map
 (define-derived-mode mercury-mode prolog-mode "Prolog[Mercury]"
   "Major mode for editing Mercury programs.
 Actually this is just customized `prolog-mode'."
-  (setq-local prolog-system 'mercury))
+  (setq-local prolog-system 'mercury)
+  (prolog-mode-variables))
 
 
 ;;-------------------------------------------------------------------
@@ -2082,7 +2083,8 @@ prolog-font-lock-keywords
     (delq
      nil
      (cond
-      ((eq major-mode 'prolog-mode)
+      ((or (eq major-mode 'prolog-mode)
+           (eq major-mode 'mercury-mode))
        (list
         head-predicates
         head-predicates-1