Summary form only given. Web services present a promising software technology, which provides application-to-application interaction. They are based on communication protocols, service descriptions, and service discovery and are built on top of existing Web protocols and based on open XML standards. Web services are described using Web Services Description Language (WSDL), and the universal description, discovery, and integration directory provide a registry of Web services descriptions. Testing Web services is important for both the Web service provider and the Web service user. This paper proposes a technique for testing Web services using mutation analysis. The technique is based on applying mutation operators to the WSDL document in order to generate mutated Web service interfaces that are used to test the Web service. For this purpose, we define mutant operators that are specific to WSDL documents. Our empirical results have shown the usefulness of this technique.