diff --git a/run_tests b/run_tests
index 6c6f859..573dd44 100755
--- a/run_tests
+++ b/run_tests
@@ -24,6 +24,10 @@
 
 def find_pytest():
   """Try to locate a good version of pytest."""
+  # If we're in a virtualenv, assume that it's provided the right pytest.
+  if 'VIRTUAL_ENV' in os.environ:
+    return 'pytest'
+
   # Use the Python 3 version if available.
   ret = shutil.which('pytest-3')
   if ret: