Hello, I'm using MPI_Comm_Spawn to spawn processes dynamically. I spawn few processes, then I execute something, then I spawn few more processes. I want to merge intracommunicators in such way that the master Processes including all the spawn children processes (the first and Second time) would have rank 0, 1, 2, 3, .... p-1. My problem is that I cannot merge intracommunicators. Here is what I mean: Parent code: =========== error = MPI_Comm_spawn("child", MPI_ARGV_NULL, 2, info, 0, MPI_COMM_WORLD, &intercomm, errcodes); MPI_Intercomm_merge(intercomm, 0, &intracomm); ..... do something .... error = MPI_Comm_spawn("child", MPI_ARGV_NULL, 3, info, 0, MPI_COMM_WORLD, &intercomm, errcodes); MPI_Intercomm_merge(intercomm, 0, &intracomm); The child code: =============== MPI_Comm_get_parent(&intercomm); MPI_Comm_size(intercomm,&nprocs); printf(" child nprocs = %d\n", nprocs); MPI_Intercomm_merge(intercomm, 0, &intracomm); MPI_Comm_rank(intracomm, &rank); printf(" child rank = %d\n", rank); ===== The outptut after executing: mpiexec -n 1 ./parent Gives: child rank 1, 2 (for the first spawn) Child rank 1, 2, 3 (for the second spawn) The question is that how I can merge so that my processes would have ranks 1, 2, 3, 4, 5? Thanks a lot, Ali