diff --git a/master/custom/workers.py b/master/custom/workers.py
index fcf24bc3..bb282dd5 100644
--- a/master/custom/workers.py
+++ b/master/custom/workers.py
@@ -282,7 +282,9 @@ def get_workers(settings):
             name="rkm-arm64-ios-simulator",
             tags=['iOS'],
             not_branches=['3.9', '3.10', '3.11', '3.12'],
-            parallel_builders=4,
+            # Starting/running simulators in parallel can be problematic;
+            # see python/cpython#129200
+            parallel_builders=1,
         ),
         cpw(
             name="mhsmith-android-aarch64",