diff --git a/tox.ini b/tox.ini
index 56dbb72..480a6d6 100644
--- a/tox.ini
+++ b/tox.ini
@@ -10,7 +10,7 @@ setenv = VIRTUAL_ENV={envdir}
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 commands =
-  python setup.py testr --testr-args='{posargs}'
+  python setup.py testr --slowest --testr-args='{posargs}'
 
 [tox:jenkins]
 downloadcache = ~/cache/pip