diff --git a/Data/Aeson/Types.hs b/Data/Aeson/Types.hs
--- a/Data/Aeson/Types.hs
+++ b/Data/Aeson/Types.hs
@@ -64,6 +64,7 @@
 import qualified Data.ByteString as B
 import qualified Data.ByteString.Lazy as LB
 import qualified Data.HashMap.Strict as H
+import qualified Data.HashSet as HashSet
 import qualified Data.IntSet as IntSet
 import qualified Data.Map as M
 import qualified Data.Set as Set
@@ -567,6 +568,14 @@
     
 instance (Ord a, FromJSON a) => FromJSON (Set.Set a) where
     parseJSON = fmap Set.fromList . parseJSON
+    {-# INLINE parseJSON #-}
+
+instance (ToJSON a) => ToJSON (HashSet.HashSet a) where
+    toJSON = toJSON . HashSet.toList
+    {-# INLINE toJSON #-}
+    
+instance (Eq a, Hashable a, FromJSON a) => FromJSON (HashSet.HashSet a) where
+    parseJSON = fmap HashSet.fromList . parseJSON
     {-# INLINE parseJSON #-}
 
 instance ToJSON IntSet.IntSet where
diff --git a/aeson.cabal b/aeson.cabal
--- a/aeson.cabal
+++ b/aeson.cabal
@@ -1,5 +1,5 @@
 name:            aeson
-version:         0.3.2.7
+version:         0.3.2.8
 license:         BSD3
 license-file:    LICENSE
 category:        Text, Web, JSON