diff --git a/Data/Supply.hs b/Data/Supply.hs
--- a/Data/Supply.hs
+++ b/Data/Supply.hs
@@ -34,10 +34,9 @@
   , split4
   ) where
 
--- Using 'MVar's might be a bit heavy but it ensures that
--- multiple threads that share a supply will get distinct names.
-import Control.Concurrent.MVar
-import System.IO.Unsafe(unsafePerformIO)
+-- Usinga an IORef is thread-safe because we update it with 'atomicModifyIORef'.
+import Data.IORef
+import System.IO.Unsafe(unsafeInterleaveIO)
 
 -- Basics ----------------------------------------------------------------------
 
@@ -66,20 +65,16 @@
 -- the first argument is an initial value, the
 -- second specifies how to generate a new value from an existing one.
 newSupply    :: a -> (a -> a) -> IO (Supply a)
-newSupply x f = fmap (gen True) (newMVar (iterate f x))
-
-  -- The extra argument to ``gen'' is passed because without
-  -- it Hugs spots that the recursive calls are the same but does
-  -- not know that unsafePerformIO is unsafe.
-  where gen _ r = Node { supplyValue  = unsafePerformIO (genSym r),
-                         supplyLeft   = gen False r,
-                         supplyRight  = gen True r }
+newSupply x f = gen =<< newIORef (iterate f x)
 
-        genSym       :: MVar [a] -> IO a
-        genSym r      = do a : as <- takeMVar r
-                           putMVar r as
-                           return a
+  where gen r = do v <- unsafeInterleaveIO (genSym r)
+                   ls <- unsafeInterleaveIO (gen r)
+                   rs <- unsafeInterleaveIO (gen r)
+                   return (Node v ls rs)
 
+        genSym       :: IORef [a] -> IO a
+        genSym r      = atomicModifyIORef r (\(a:as) -> (as,a))
+                           
 -- | Generate a new supply by systematically applying a function
 -- to an existing supply.  This function, together with 'supplyValue'
 -- form a comonad on 'Supply'.
diff --git a/value-supply.cabal b/value-supply.cabal
--- a/value-supply.cabal
+++ b/value-supply.cabal
@@ -1,5 +1,5 @@
 Name:           value-supply
-Version:        0.1
+Version:        0.2
 License:        BSD3
 License-file:   LICENSE
 Author:         Iavor S. Diatchki