diff --git a/src/serial_io.c b/src/serial_io.c
index 7daf9aeaa2e1d531d5a910446841f504ff24b151..92a576da8588a4b66707de11d002d0636139492a 100644
--- a/src/serial_io.c
+++ b/src/serial_io.c
@@ -445,7 +445,7 @@ void write_output (struct engine *e, struct UnitSystem* us)
     error("Error while creating file header\n");
     
   /* Print the relevant information and print status */
-  writeAttribute(h_grp, "BoxSize", DOUBLE, e->s->dim, 1);
+  writeAttribute(h_grp, "BoxSize", DOUBLE, e->s->dim, 3);
   writeAttribute(h_grp, "NumPart_ThisFile", UINT, numParticles, 6);
   writeAttribute(h_grp, "Time", DOUBLE, &e->time, 1);