diff --git a/src/mesh/python/x3d_generator.py b/src/mesh/python/x3d_generator.py
index fb8869eba..1344b510a 100755
--- a/src/mesh/python/x3d_generator.py
+++ b/src/mesh/python/x3d_generator.py
@@ -187,6 +187,11 @@
     np.savetxt(fname + '.bdy' + str(i + 1) + '.in',
                np.unique(np.array(mesh.nodes_per_side[i]) + 1), fmt='%d')
 
+# ------------------------------------------------------------------------------------------------ #
+# -- write out the mesh region files based on matid
+# -- todo: map (dictionary) matid to cell lists (this is only for X3D, so it's not a mesh task)
+np.savetxt(fname + '.Reg' + str(1) + '.in', np.arange(1, mesh.num_cells + 1), fmt='%d')
+
 # ------------------------------------------------------------------------------------------------ #
 # end of x3d_generator.py
 # ------------------------------------------------------------------------------------------------ #