Abstract. A basic prerequisite for parallel programming is a good communication API. The recent interest in using Java for scienti c and engineering application has led to several international e orts to produce a message passing interface to support parallel computation. In this paper we describe and then discuss the syntax, functionality and performance of one such interface, mpiJava, an object-oriented Java interface to MPI. We rst discuss the design of the mpiJava API and the issues associated with its development. We then move on to brie y outline the steps necessary to 'port' mpiJava onto a range of operating systems, including Windows NT, Linux and Solaris. In the second part of the paper we present and then discuss some performance measurements made of communications bandwidth and latency to compare mpiJava on these systems. Finally, we summarise our experiences and then brie y mention work that we plan to undertake.