diff --git a/fix-parser-simple.cabal b/fix-parser-simple.cabal
--- a/fix-parser-simple.cabal
+++ b/fix-parser-simple.cabal
@@ -1,5 +1,5 @@
 Name:fix-parser-simple
-Version:15320.2
+Version:15320.3
 Description:Simple fix-expression parser
 License:LGPL
 License-File:license.txt
@@ -10,8 +10,15 @@
 Build-Type:Simple
 Cabal-Version: >= 1.2
 
+Flag use_mtl {
+  Description: Use mtl rather than mmtl
+  Default:     False
+}
+
 Library {
-  Build-Depends: base >= 4 && < 5, mmtl
+  Build-Depends: base >= 4 && < 5
+  if flag(use_mtl) { Build-Depends:  mtl }
+  else             { Build-Depends: mmtl }
   Extensions: PatternGuards
   Exposed-Modules: Parser.Fix.Simple
   Other-Modules: Util