The MPJ programming interface has been defined by the Java Grande forum to provide MPI-like message passing for Java applications. In this paper, we present MPJ/Ibis, the first implementation of MPJ based on our Ibis programming environment for cluster and grid computing. By exploiting both flexibility and efficiency of Ibis, our MPJ implementation delivers high-performance communication, while being deployable on various platforms, from Myrinet-based clusters to grids. We evaluated MPJ/Ibis on our DAS-2 cluster. Our results show that MPJ/Ibis’ performance is competitive to mpiJava on Myrinet and Fast Ethernet, and to C-based MPICH on Fast Ethernet.