diff --git a/tox.ini b/tox.ini
index 9281e78ab..028453f36 100644
--- a/tox.ini
+++ b/tox.ini
@@ -13,7 +13,14 @@ usedevelop = True
 install_command = pip install -U {opts} {packages}
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
-commands = python setup.py testr --testr-args='{posargs}'
+# Delete bytecodes from normal directories before running tests.
+# Note that bytecodes in dot directories will not be deleted
+# to keep bytecodes of python modules installed into virtualenvs.
+commands = sh -c "find . -type d -name '.?*' -prune -o \
+           \( -type d -name '__pycache__' -o -type f -name '*.py[co]' \) \
+           -print0 | xargs -0 rm -rf"
+           python setup.py testr --testr-args='{posargs}'
+whitelist_externals = sh
 
 [testenv:pep8]
 commands = flake8