ASP.NET Web Services use the .asmx file extension, which stands for ASp.NET neXt generation (or 'asmx'). The .aspx extension is for web pages, .ascx is for user controls, .wsdl is for Web Service Description Language files, and .svc is for WCF services. When you create an ASP.NET Web Service in Visual Studio, it automatically generates an .asmx file.