diff --git a/tox.ini b/tox.ini
index af7aa4f..dfa7860 100644
--- a/tox.ini
+++ b/tox.ini
@@ -42,7 +42,9 @@ commands =
   towncrier --version {posargs} --yes
 
 [pytest]
-addopts = -ra
+# pytest-services also defines a worker_id fixture, disable
+# it so they don't conflict with each other (#611).
+addopts = -ra -p no:pytest-services
 testpaths = testing
 
 [flake8]