diff --git a/tox.ini b/tox.ini
index 905747f..253b00b 100644
--- a/tox.ini
+++ b/tox.ini
@@ -17,7 +17,7 @@
     PIP_NO_BINARY=cryptography
 commands =
     openssl version
-    coverage run --parallel -m pytest {posargs}
+    coverage run --parallel -m pytest -x {posargs}
 
 [testenv:py27-twistedMaster]
 deps =