diff --git a/lib/cli/run.js b/lib/cli/run.js
index 9c9911131d..4b59a13ecd 100644
--- a/lib/cli/run.js
+++ b/lib/cli/run.js
@@ -87,7 +87,7 @@ exports.builder = yargs =>
       },
       extension: {
         default: defaults.extension,
-        defaultDescription: 'js',
+        defaultDescription: 'js,cjs,mjs',
         description: 'File extension(s) to load',
         group: GROUPS.FILES,
         requiresArg: true,
diff --git a/lib/mocharc.json b/lib/mocharc.json
index e6f5d99c5b..1ed9157675 100644
--- a/lib/mocharc.json
+++ b/lib/mocharc.json
@@ -1,6 +1,6 @@
 {
   "diff": true,
-  "extension": ["js"],
+  "extension": ["js", "cjs", "mjs"],
   "opts": "./test/mocha.opts",
   "package": "./package.json",
   "reporter": "spec",