diff --git a/csg2csg/MCNPInput.py b/csg2csg/MCNPInput.py
index 142923f..1b0b5ae 100644
--- a/csg2csg/MCNPInput.py
+++ b/csg2csg/MCNPInput.py
@@ -98,7 +98,7 @@ def __get_importances(self, start_line):
                 return self.__process_importances()
 
             # check for importance keyword
-            if "imp" in self.file_lines[idx]:
+            if self.file_lines[idx].startswith('imp'):
                 particle = self.file_lines[idx].split()[0].split(":")[1]
 
                 # TODO mcnp allows the following forms imp:n imp:n,p etc