# ParallelizationTrait A type that defines whether the testing library runs this test serially or in parallel. ``` struct ParallelizationTrait ``` ## Overview When you add this trait to a parameterized test function, that test runs its cases serially instead of in parallel. This trait has no effect when you apply it to a non-parameterized test function. When you add this trait to a test suite, that suite runs its contained test functions (including their cases, when parameterized) and sub-suites serially instead of in parallel. If the sub-suites have children, they also run serially. This trait does not affect the execution of a test relative to its peers or to unrelated tests. This trait has no effect if you disable test parallelization globally (for example, by passing `--no-parallel` to the `swift test` command.) To add this trait to a test, use [`serialized`](/documentation/Testing/Trait/serialized). --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)