diff --git a/openmc/mgxs/mgxs.py b/openmc/mgxs/mgxs.py
index 7c2156c196..7f34f8856f 100644
--- a/openmc/mgxs/mgxs.py
+++ b/openmc/mgxs/mgxs.py
@@ -47,13 +47,15 @@ _DOMAINS = [openmc.Cell,
 
 
 class MGXS(object):
-    """A multi-group cross section for some energy group structure within
-    some spatial domain.
+    """An abstract multi-group cross section for some energy group structure 
+    within some spatial domain.
 
     This class can be used for both OpenMC input generation and tally data
     post-processing to compute spatially-homogenized and energy-integrated
     multi-group cross sections for deterministic neutronics calculations.
 
+    NOTE: Users should instantiate the subclasses of this abstract class.
+
     Parameters
     ----------
     domain : Material or Cell or Universe