diff --git a/actionview/test/abstract_unit.rb b/actionview/test/abstract_unit.rb
index d5d27b9335..600bb94df3 100644
--- a/actionview/test/abstract_unit.rb
+++ b/actionview/test/abstract_unit.rb
@@ -179,7 +179,11 @@ module ActionDispatch
 end
 
 class ActiveSupport::TestCase
-  parallelize
+  if Process.respond_to?(:fork) && !Gem.win_platform?
+    parallelize
+  else
+    parallelize(with: :threads)
+  end
 
   include ActiveSupport::Testing::MethodCallAssertions
 
diff --git a/activesupport/test/abstract_unit.rb b/activesupport/test/abstract_unit.rb
index cedfd60974..c349d16688 100644
--- a/activesupport/test/abstract_unit.rb
+++ b/activesupport/test/abstract_unit.rb
@@ -28,7 +28,11 @@ ActiveSupport.to_time_preserves_timezone = ENV["PRESERVE_TIMEZONES"] == "1"
 I18n.enforce_available_locales = false
 
 class ActiveSupport::TestCase
-  parallelize
+  if Process.respond_to?(:fork) && !Gem.win_platform?
+    parallelize
+  else
+    parallelize(with: :threads)
+  end
 
   include ActiveSupport::Testing::MethodCallAssertions