diff --git a/.github/workflows/tox.yml b/.github/workflows/tox.yml
index 308fab11..a2a013eb 100644
--- a/.github/workflows/tox.yml
+++ b/.github/workflows/tox.yml
@@ -14,7 +14,11 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ubuntu-latest, macos-latest] # All OSes pass except Windows because tests need Unix-only fcntl, grp, pwd, etc.
+        os:
+         - ubuntu-latest
+         # not defaulting to macos-latest: Python <= 3.9 was missing from macos-14 @ arm64
+         - macos-13
+         # Not testing Windows, because tests need Unix-only fcntl, grp, pwd, etc.
         python-version:
          # CPython <= 3.7 is EoL since 2023-06-27
          - "3.7"
@@ -26,6 +30,11 @@ jobs:
          # PyPy <= 3.8 is EoL since 2023-06-16
          - "pypy-3.9"
          - "pypy-3.10"
+        include:
+         # Note: potentially "universal2" release
+         # https://github.com/actions/runner-images/issues/9741
+         - os: macos-latest
+           python-version: "3.12"
     steps:
       - uses: actions/checkout@v4
       - name: Using Python ${{ matrix.python-version }}