diff --git a/tox.ini b/tox.ini
index 93c48874..a426a7b6 100644
--- a/tox.ini
+++ b/tox.ini
@@ -137,6 +137,7 @@ commands =
 # flake8 finds any problems.
 [testenv:flake8]
 description = Lint code with flake8.
+basepython = python3.9
 changedir = {toxinidir}
 deps = flake8
 commands =
@@ -146,6 +147,7 @@ commands =
 # any problems are found.
 [testenv:isort]
 description = Lint imports with isort.
+basepython = python3.9
 changedir = {toxinidir}
 deps = isort
 commands =