diff --git a/autotest/conftest.py b/autotest/conftest.py
index a9d817c8d..b85458c04 100644
--- a/autotest/conftest.py
+++ b/autotest/conftest.py
@@ -118,6 +118,14 @@ def pytest_addoption(parser):
         "but automated tests should probably also check patch collections or figure & axis properties.)",
     )
 
+    # for test_generate_classes.py
+    parser.addoption(
+        "--ref",
+        action="append",
+        type=str,
+        help="Include extra refs to test. Useful for testing branches on a fork, e.g. <your GitHub username>/modflow6/<your branch>.",
+    )
+
 
 def pytest_report_header(config):
     """Header for pytest to show versions of packages."""
diff --git a/autotest/test_generate_classes.py b/autotest/test_generate_classes.py
index 06527a56f..e48c73eb1 100644
--- a/autotest/test_generate_classes.py
+++ b/autotest/test_generate_classes.py
@@ -1,15 +1,62 @@
 import sys
+from datetime import datetime
+from os import environ
 from pathlib import Path
 from pprint import pprint
+from typing import Iterable
+from warnings import warn
 
 import pytest
 
 
+def nonempty(itr: Iterable):
+    for x in itr:
+        if x:
+            yield x
+
+
+def pytest_generate_tests(metafunc):
+    # defaults
+    ref = [
+        "MODFLOW-USGS/modflow6/develop",
+        "MODFLOW-USGS/modflow6/master",
+        "MODFLOW-USGS/modflow6/6.4.1",
+    ]
+
+    # refs provided as env vars override the defaults
+    ref_env = environ.get("TEST_GENERATE_CLASSES_REF")
+    if ref_env:
+        ref = nonempty(ref_env.strip().split(","))
+
+    # refs given as CLI options override everything
+    ref_opt = metafunc.config.getoption("--ref")
+    if ref_opt:
+        ref = nonempty([o.strip() for o in ref_opt])
+
+    # drop duplicates
+    ref = list(dict.fromkeys(ref))
+
+    # drop and warn refs with invalid format
+    # i.e. not "owner/repo/branch"
+    for r in ref:
+        spl = r.split("/")
+        if len(spl) != 3 or not all(spl):
+            warn(f"Skipping invalid ref: {r}")
+            ref.remove(r)
+
+    key = "ref"
+    if key in metafunc.fixturenames:
+        metafunc.parametrize(key, ref, scope="session")
+
+
 @pytest.mark.mf6
-def test_generate_classes_from_dfn(virtualenv, project_root_path):
+@pytest.mark.slow
+def test_generate_classes_from_dfn(virtualenv, project_root_path, ref):
     python = virtualenv.python
     venv = Path(python).parent
-    print(f"Using temp venv at {venv} with python {python}")
+    print(
+        f"Using temp venv at {venv} with python {python} to test class generation from {ref}"
+    )
 
     # install flopy/dependencies
     pprint(virtualenv.run(f"pip install {project_root_path}"))
@@ -31,9 +78,10 @@ def test_generate_classes_from_dfn(virtualenv, project_root_path):
     mod_file_times = [Path(mod_file).stat().st_mtime for mod_file in mod_files]
     pprint(mod_files)
 
-    # generate classes from develop branch
-    owner = "MODFLOW-USGS"
-    branch = "develop"
+    # generate classes
+    spl = ref.split("/")
+    owner = spl[0]
+    branch = spl[2]
     pprint(
         virtualenv.run(
             "python -c 'from flopy.mf6.utils import generate_classes; generate_classes(owner=\""
@@ -44,31 +92,25 @@ def test_generate_classes_from_dfn(virtualenv, project_root_path):
         )
     )
 
+    def get_mtime(f):
+        try:
+            return Path(f).stat().st_mtime
+        except:
+            return 0  # if file not found
+
     # make sure files were regenerated
     modified_files = [
         mod_files[i]
         for i, (before, after) in enumerate(
             zip(
                 mod_file_times,
-                [Path(mod_file).stat().st_mtime for mod_file in mod_files],
+                [get_mtime(f) for f in mod_files],
             )
         )
-        if after > before
+        if after > 0 and after > before
     ]
     assert any(modified_files)
     print(f"{len(modified_files)} files were modified:")
     pprint(modified_files)
 
-    # try with master branch
-    branch = "master"
-    pprint(
-        virtualenv.run(
-            "python -c 'from flopy.mf6.utils import generate_classes; generate_classes(owner=\""
-            + owner
-            + '", branch="'
-            + branch
-            + "\", backup=False)'"
-        )
-    )
-
     # todo checkout mf6 and test with dfnpath? test with backups?