Access to shared data is critical to the long term success of grids of distributed systems. As more parallel applications are being used on these grids, the need for some kind of parallel I/O facility across distributed systems increases. However, grid middleware has thus far had only limited support for distributed parallel I/O. In this paper, we present an implementation of the MPI-2 I/O interface using the Globus GridFTP client API. MPI is widely used for parallel computing, and its I/O interface maps onto a large variety of storage systems. The limitations of using GridFTP as an MPI-I/O transport mechanism are described, as well as support for parallel access to scientific data formats such as HDF and NetCDF. We compare the performance of GridFTP to that of NFS on the same network using several parallel I/O benchmarks. Our tests indicate that GridFTP can be a workable transport for parallel I/O, particularly for distributed read-only access to shared data sets.