diff --git a/mes/module/mes/scm.mes b/mes/module/mes/scm.mes
index 25dcb2fa..3ca92ca1 100644
--- a/mes/module/mes/scm.mes
+++ b/mes/module/mes/scm.mes
@@ -194,9 +194,6 @@
 (define (make-string n . fill)
   (list->string (apply make-list n fill)))
 
-(define (string-length s)
-  (length (string->list s)))
-
 (define (string-ref s k)
   (list-ref (string->list s) k))
 
diff --git a/src/strings.c b/src/strings.c
index 11630a26..9073c2a9 100644
--- a/src/strings.c
+++ b/src/strings.c
@@ -260,3 +260,10 @@ string_append (SCM x) ///((arity . n))
     }
   return make_string (buf, size);
 }
+
+SCM
+string_length (SCM string)
+{
+  assert (TYPE (string) == TSTRING);
+  return MAKE_NUMBER (LENGTH (string));
+}