MPI is a de facto standard for high performance numerical applications on parallel machines: it is available, in a variety of implementations, for a range of architectures, ranging from supercomputers to clusters of workstations. Nonetheless, with the growing demand for distributed, heterogeneous and Grid computing, developers are hitting some of its limitations: e.g. security is not addressed, and geographically distributed machines are difficult to connect. In this work, we give an example of a parallel application, implemented with the use of Web Services. Web Services represent an emerging standard to offer computational services over the Internet. While this solution does not reach the same performance of MPI, it offers a series of advantages: high availability, rapid design, extreme heterogeneity.