diff --git a/nc_test4/run_par_test.sh.in b/nc_test4/run_par_test.sh.in
index 00e4db3631..8ad53b2d5e 100644
--- a/nc_test4/run_par_test.sh.in
+++ b/nc_test4/run_par_test.sh.in
@@ -52,7 +52,6 @@ echo "Parallel I/O test for Collective I/O, contributed by HDF Group."
 if test "@HAS_PAR_FILTERS@" = "yes"; then
     echo
     echo "Parallel I/O test with zlib."
-    @MPIEXEC@ -n 1 ./tst_parallel_zlib
     @MPIEXEC@ -n 4 ./tst_parallel_zlib
 
     echo
diff --git a/nc_test4/tst_parallel_zlib.c b/nc_test4/tst_parallel_zlib.c
index 3a7906ef94..7c25cfee4a 100644
--- a/nc_test4/tst_parallel_zlib.c
+++ b/nc_test4/tst_parallel_zlib.c
@@ -60,6 +60,10 @@ main(int argc, char **argv)
     /*printf("mpi_name: %s size: %d rank: %d\n", mpi_name,
       mpi_size, mpi_rank);*/
 
+    /* This program must have exactly 4 processors. */
+    if (mpi_size != 4)
+	ERR;
+
 #ifdef USE_MPE
     MPE_Init_log();
     s_init = MPE_Log_get_event_number();