diff --git a/openmc/surface.py b/openmc/surface.py
index 62ea83456c..a4c8b8c88d 100644
--- a/openmc/surface.py
+++ b/openmc/surface.py
@@ -24,8 +24,6 @@ _WARNING_KWARGS = """\
 will not accept positional parameters for superclass arguments.\
 """
 
-_SURFACE_CLASSES = {}
-
 
 class Surface(IDManagerMixin, metaclass=ABCMeta):
     """An implicit surface with an associated boundary condition.