In mpich2-1.0.8p1/src/mpi/coll/alltoallw.c, lines 361 - 372, error checking is not necessary for those sendtypes[] and recvtypes[] when their sendcnts[] and recvcnts[] are zeros. I got error messages from these checking when a subset of processes has zero-length to send or receive, and I used MPI_DATATYPE_NULL for those cases. Wei-keng