The paper discusses use of the Java object serialization model for marshalling communication data in Java bindings of the Message Passing Interface standard, MPI. This approach is compared with a Java transcription of the derived datatype mechanism used for buffer description in C and Fortran bindings of MPI. We describe an implementation of the mpiJava interface to MPI incorporating automatic object serialization. The programming model is attractive, though benchmark results show that the current JDK implementation of serialization is not fast enough for high performance applications. Strategies to improve this situation are discussed.