diff --git a/run-tests.php b/run-tests.php
index b83bac173ead7f0f72e192780ebb7b221d76d6aa..50237a8ae9939387d679c86972b9c7aca4404f36 100644
--- a/run-tests.php
+++ b/run-tests.php
@@ -10,6 +10,7 @@ $help_strings = ['-h', '--help', '-v', '--version'];
 if (!$site_to_test || in_array($site_to_test, $help_strings)) {
   $script = basename($_SERVER['argv'][0]);
   echo 'Usage: ' . $script . " URL-TO-TEST [ADDITIONAL-ARGUMENTS ...]\n";
+  echo "To test specified classes, use: --class CLASSNAME[,CLASSNAME]\n";
   echo "In Drupal 8, pass the argument --suppress-deprecations.\n";
   echo "(The argument is not needed for Drupal 9.)\n";
   exit(1);
@@ -33,11 +34,14 @@ $_SERVER['argv'] = [
   '--non-html',
   '--sqlite',
   '/tmp/drupal/test.sqlite',
-  '--module',
-  implode(',', $modules_to_test),
   '--url',
   $site_to_test,
 ];
+// Allow testing specified classes. Default to running all module tests.
+if (!array_search('--class', $original_argv)) {
+  $_SERVER['argv'][] = '--module';
+  $_SERVER['argv'][] = implode(',', $modules_to_test);
+}
 $_SERVER['argv'] = array_merge($_SERVER['argv'], $original_argv);
 
 // Run the tests.