diff --git a/packages/core/cyclist.mjs b/packages/core/cyclist.mjs
index a8ccd30f..819c31bc 100644
--- a/packages/core/cyclist.mjs
+++ b/packages/core/cyclist.mjs
@@ -4,7 +4,7 @@ Copyright (C) 2022 Strudel contributors - see <https://github.com/tidalcycles/st
 This program is free software: you can redistribute it and/or modify it under the terms of the GNU Affero General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Affero General Public License for more details. You should have received a copy of the GNU Affero General Public License along with this program.  If not, see <https://www.gnu.org/licenses/>.
 */
 
-import createClock from './zyklus';
+import createClock from './zyklus.mjs';
 import { logger } from './logger.mjs';
 
 export class Cyclist {
diff --git a/packages/core/zyklus.js b/packages/core/zyklus.mjs
similarity index 100%
rename from packages/core/zyklus.js
rename to packages/core/zyklus.mjs