diff --git a/build.js b/build.js
index 23f0d1df..cbd2322e 100644
--- a/build.js
+++ b/build.js
@@ -55,6 +55,6 @@ makeBundle(
             })
             .join("\n");
 
-        fs.writeFileSync("pkg/fake-timers-esm.js", source);
+        fs.writeFileSync("pkg/fake-timers-esm.mjs", source);
     }
 );
diff --git a/package.json b/package.json
index 83c7938b..04102e06 100644
--- a/package.json
+++ b/package.json
@@ -20,7 +20,7 @@
     "test-check-coverage": "npm run test-coverage && nyc check-coverage",
     "test-cloud": "mochify --wd --no-detect-globals --timeout=10000",
     "test-coverage": "nyc --all --reporter text --reporter html --reporter lcovonly npm run test-node",
-    "test-esm": "node -r esm test-esm/fake-timers-esm-test.js",
+    "test-esm": "node test-esm/fake-timers-esm-test.mjs",
     "test": "npm run lint && npm run test-node && npm run test-headless && npm run test-esm",
     "bundle": "node ./build.js",
     "prepublishOnly": "npm run bundle",
@@ -86,7 +86,7 @@
       }
     ]
   },
-  "module": "./pkg/fake-timers-esm.js",
+  "module": "./pkg/fake-timers-esm.mjs",
   "cdn": "./pkg/fake-timers.js",
   "jsdelivr": "./pkg/fake-timers.js",
   "main": "./src/fake-timers-src.js",
@@ -105,7 +105,7 @@
     "statements": 92,
     "exclude": [
       "**/*-test.js",
-      "**/*-test-esm.js",
+      "test-esm/**",
       "build.js",
       "coverage/**",
       "pkg/**"
diff --git a/test-esm/fake-timers-esm-test.js b/test-esm/fake-timers-esm-test.mjs
similarity index 89%
rename from test-esm/fake-timers-esm-test.js
rename to test-esm/fake-timers-esm-test.mjs
index a6dd1181..51bb492e 100644
--- a/test-esm/fake-timers-esm-test.js
+++ b/test-esm/fake-timers-esm-test.mjs
@@ -1,5 +1,5 @@
 /* eslint-disable no-console */
-import * as timers from "../pkg/fake-timers-esm.js";
+import * as timers from "../pkg/fake-timers-esm.mjs";
 
 const expectedExports = ["timers", "createClock", "install", "withGlobal"];