diff --git a/buildspec-release.yml b/buildspec-release.yml
index 18474a1..90ab150 100644
--- a/buildspec-release.yml
+++ b/buildspec-release.yml
@@ -17,7 +17,7 @@ phases:
       - tox -e black-check
 
       # run unit tests
-      - tox -e py27,py36,py37 --parallel all -- test/unit
+      - tox -e py27,py36,py37 -- test/unit
 
       # build dummy container
       - python3 setup.py sdist
diff --git a/buildspec.yml b/buildspec.yml
index 8f8ea69..b11ce9e 100644
--- a/buildspec.yml
+++ b/buildspec.yml
@@ -15,7 +15,7 @@ phases:
       - tox -e twine
 
       # run unit tests
-      - tox -e py27,py36,py37 --parallel all -- test/unit
+      - tox -e py27,py36,py37 -- test/unit
 
       # build dummy container
       - python setup.py sdist