diff --git a/tox.ini b/tox.ini
index a1e0ac70d3..58cf9613cb 100644
--- a/tox.ini
+++ b/tox.ini
@@ -6,6 +6,8 @@ deps =
     coverage
     Twisted>=15.1
     mock
+setenv =
+    PYTHONDONTWRITEBYTECODE = no_byte_code
 commands =
     coverage run --source=./synapse {envbindir}/trial tests
     coverage report -m