diff --git a/exercise3/wave_1d_parallel.c b/exercise3/wave_1d_parallel.c
index ef9847c..2aaa42d 100644
--- a/exercise3/wave_1d_parallel.c
+++ b/exercise3/wave_1d_parallel.c
@@ -50,6 +50,8 @@ real_t
 // Save the present time step in a numbered file under 'data/'.
 void domain_save(int_t step) {
   // BEGIN: T8
+  if (comm_rank != ROOT)
+    return;
   char filename[256];
   sprintf(filename, "data/%.5ld.dat", step);
   FILE *out = fopen(filename, "wb");