diff --git a/pyproject.toml b/pyproject.toml
index a9b8fc141..23afbf19b 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -109,6 +109,7 @@ markers = [
     "adsim: require the ADsim IOC to be running",
 ]
 asyncio_mode = "auto"
+asyncio_default_fixture_loop_scope = "function"
 
 [tool.coverage.run]
 data_file = "/tmp/ophyd_async.coverage"