diff --git a/lsm-tree.cabal b/lsm-tree.cabal
index 035d60205..4f44ef253 100644
--- a/lsm-tree.cabal
+++ b/lsm-tree.cabal
@@ -602,6 +602,7 @@ test-suite lsm-tree-test
     Test.Database.LSMTree.Internal.WriteBufferBlobs.FS
     Test.Database.LSMTree.Internal.WriteBufferReader.FS
     Test.Database.LSMTree.Model.Table
+    Test.Database.LSMTree.Parallel
     Test.Database.LSMTree.Resolve
     Test.Database.LSMTree.StateMachine
     Test.Database.LSMTree.StateMachine.DL
diff --git a/src/Database/LSMTree/Internal/Primitive.hs b/src/Database/LSMTree/Internal/Primitive.hs
index bbe8d82da..51830c7e5 100644
--- a/src/Database/LSMTree/Internal/Primitive.hs
+++ b/src/Database/LSMTree/Internal/Primitive.hs
@@ -3,7 +3,9 @@
 {-# OPTIONS_HADDOCK not-home #-}
 
 module Database.LSMTree.Internal.Primitive (
-    indexWord8ArrayAsWord16
+    byteSwapInt
+  , indexWord8ArrayAsInt
+  , indexWord8ArrayAsWord16
   , indexWord8ArrayAsWord32
   , indexWord8ArrayAsWord64
   ) where
@@ -12,6 +14,15 @@ import           Data.Primitive.ByteArray (ByteArray (..))
 import           GHC.Exts
 import           GHC.Word
 
+{-# INLINE byteSwapInt #-}
+byteSwapInt :: Int -> Int
+byteSwapInt (I# i#) = I# (word2Int# (byteSwap# (int2Word# i#)))
+
+{-# INLINE indexWord8ArrayAsInt #-}
+indexWord8ArrayAsInt :: ByteArray -> Int -> Int
+indexWord8ArrayAsInt (ByteArray !ba#) (I# !off#) =
+  I# (indexWord8ArrayAsInt# ba# off#)
+
 {-# INLINE indexWord8ArrayAsWord16 #-}
 indexWord8ArrayAsWord16 :: ByteArray -> Int -> Word16
 indexWord8ArrayAsWord16 (ByteArray !ba#) (I# !off#) =
diff --git a/src/Database/LSMTree/Internal/Serialise/Class.hs b/src/Database/LSMTree/Internal/Serialise/Class.hs
index 2a31fb1b4..cc3ebd0e5 100644
--- a/src/Database/LSMTree/Internal/Serialise/Class.hs
+++ b/src/Database/LSMTree/Internal/Serialise/Class.hs
@@ -28,7 +28,7 @@ import qualified Data.Vector.Primitive as VP
 import           Data.Void (Void, absurd)
 import           Data.Word
 import           Database.LSMTree.Internal.ByteString (byteArrayToSBS)
-import           Database.LSMTree.Internal.Primitive (indexWord8ArrayAsWord64)
+import           Database.LSMTree.Internal.Primitive
 import           Database.LSMTree.Internal.RawBytes (RawBytes (..))
 import qualified Database.LSMTree.Internal.RawBytes as RB
 import           Database.LSMTree.Internal.Vector
@@ -135,6 +135,22 @@ requireBytesExactly tyName expected actual x
       . showInt actual
       $ ""
 
+{-------------------------------------------------------------------------------
+  Int
+-------------------------------------------------------------------------------}
+
+instance SerialiseKey Int where
+  serialiseKey x = RB.RawBytes $ byteVectorFromPrim $ byteSwapInt x
+
+  deserialiseKey (RawBytes (VP.Vector off len ba)) =
+    requireBytesExactly "Int" 8 len $ byteSwapInt (indexWord8ArrayAsInt ba off)
+
+instance SerialiseValue Int where
+  serialiseValue x = RB.RawBytes $ byteVectorFromPrim $ x
+
+  deserialiseValue (RawBytes (VP.Vector off len ba)) =
+    requireBytesExactly "Int" 8 len $ indexWord8ArrayAsInt ba off
+
 {-------------------------------------------------------------------------------
   Word64
 -------------------------------------------------------------------------------}
diff --git a/test/Main.hs b/test/Main.hs
index 283debc77..6396cbe36 100644
--- a/test/Main.hs
+++ b/test/Main.hs
@@ -41,6 +41,7 @@ import qualified Test.Database.LSMTree.Internal.Vector.Growing
 import qualified Test.Database.LSMTree.Internal.WriteBufferBlobs.FS
 import qualified Test.Database.LSMTree.Internal.WriteBufferReader.FS
 import qualified Test.Database.LSMTree.Model.Table
+import qualified Test.Database.LSMTree.Parallel
 import qualified Test.Database.LSMTree.Resolve
 import qualified Test.Database.LSMTree.StateMachine
 import qualified Test.Database.LSMTree.StateMachine.DL
@@ -89,6 +90,7 @@ main = do
     , Test.Database.LSMTree.Internal.WriteBufferBlobs.FS.tests
     , Test.Database.LSMTree.Internal.WriteBufferReader.FS.tests
     , Test.Database.LSMTree.Model.Table.tests
+    , Test.Database.LSMTree.Parallel.tests
     , Test.Database.LSMTree.Resolve.tests
     , Test.Database.LSMTree.UnitTests.tests
     , Test.Database.LSMTree.StateMachine.tests
diff --git a/test/Test/Database/LSMTree/Internal/Serialise/Class.hs b/test/Test/Database/LSMTree/Internal/Serialise/Class.hs
index 86009c3bc..e9e22b5bb 100644
--- a/test/Test/Database/LSMTree/Internal/Serialise/Class.hs
+++ b/test/Test/Database/LSMTree/Internal/Serialise/Class.hs
@@ -18,7 +18,8 @@ import           Test.Tasty.QuickCheck
 
 tests :: TestTree
 tests = testGroup "Test.Database.LSMTree.Internal.Serialise.Class"
-    [ testGroup "Word64"          (allProperties @Word64 True)
+    [ testGroup "Int"             (allProperties @Int False)
+    , testGroup "Word64"          (allProperties @Word64 True)
     , testGroup "ByteString"      (allProperties @ByteString True)
     , testGroup "LazyByteString"  (allProperties @LazyByteString True)
     , testGroup "ShortByteString" (allProperties @ShortByteString True)
diff --git a/test/Test/Database/LSMTree/Parallel.hs b/test/Test/Database/LSMTree/Parallel.hs
new file mode 100644
index 000000000..612c5ffca
--- /dev/null
+++ b/test/Test/Database/LSMTree/Parallel.hs
@@ -0,0 +1,85 @@
+module Test.Database.LSMTree.Parallel (tests) where
+
+import           Control.Monad.Class.MonadAsync
+import           Control.Tracer
+import qualified Data.Map.Strict as Map
+import           Data.Semigroup
+import qualified Data.Vector as V
+import           Data.Void
+import           Database.LSMTree
+import qualified System.FS.API as FS
+import           Test.Database.LSMTree.UnitTests (ignoreBlobRef)
+import           Test.Tasty
+import           Test.Tasty.QuickCheck
+import           Test.Util.FS
+
+tests :: TestTree
+tests = testGroup "Test.Database.LSMTree.Parallel" [
+      testProperty "prop_concurrentUpserts" $
+        forAllShrink genTinyAllocNumEntries shrinkTinyAllocNumEntries
+          prop_concurrentUpserts
+    ]
+
+{-------------------------------------------------------------------------------
+  Concurrent upserts on one table
+-------------------------------------------------------------------------------}
+
+prop_concurrentUpserts ::
+     WriteBufferAlloc
+  -> ParallelShrink
+  -> V.Vector (Key, Value)
+  -> [V.Vector (Key, Value)]
+  -> V.Vector Key
+  -> Property
+prop_concurrentUpserts wbAlloc (ParallelShrink n) setupBatch parBatches lookupBatch =
+    conjoin $ replicate n $
+    ioProperty $
+    withTempIOHasBlockIO "prop_concurrentUpserts" $ \hfs hbio ->
+    withSession nullTracer hfs hbio (FS.mkFsPath []) $ \sess ->
+    withTableWith @_ @Key @Value @Blob conf sess $ \table -> do
+      upserts table setupBatch
+      forConcurrently_ parBatches $ upserts table
+      vs <- lookups table lookupBatch
+      pure $ V.map modelLookup lookupBatch === V.map ignoreBlobRef vs
+  where
+    conf = defaultTableConfig { confWriteBufferAlloc = wbAlloc }
+
+    modelTable =
+        let ms = fromBatch setupBatch : fmap fromBatch parBatches
+        in  Map.unionsWith resolve ms
+      where
+        fromBatch = Map.fromListWith resolve . V.toList
+
+    modelLookup k = case Map.lookup k modelTable of
+        Nothing -> NotFound
+        Just v  -> Found v
+
+newtype Key = Key Int
+  deriving stock (Show, Eq, Ord)
+  deriving Arbitrary via Small Int
+  deriving newtype SerialiseKey
+
+newtype Value = Value Int
+  deriving stock (Show, Eq, Ord)
+  deriving newtype SerialiseValue
+  deriving ResolveValue via ResolveViaSemigroup (Sum Int)
+  deriving Arbitrary via Int
+
+newtype Blob = Blob Void
+  deriving newtype SerialiseValue
+
+genTinyAllocNumEntries :: Gen WriteBufferAlloc
+genTinyAllocNumEntries = AllocNumEntries <$> elements [1..5]
+
+shrinkTinyAllocNumEntries :: WriteBufferAlloc -> [WriteBufferAlloc]
+shrinkTinyAllocNumEntries (AllocNumEntries x) =
+    [ AllocNumEntries x' | Positive x' <- shrink (Positive x), x' >= 2]
+
+newtype ParallelShrink = ParallelShrink Int
+  deriving stock Show
+
+instance Arbitrary ParallelShrink where
+  arbitrary = pure (ParallelShrink 1)
+  shrink (ParallelShrink n)
+    | n == 1 = [ParallelShrink 100]
+    | otherwise = []
diff --git a/test/Test/Database/LSMTree/UnitTests.hs b/test/Test/Database/LSMTree/UnitTests.hs
index 78e1e7de8..28d9a2923 100644
--- a/test/Test/Database/LSMTree/UnitTests.hs
+++ b/test/Test/Database/LSMTree/UnitTests.hs
@@ -1,7 +1,11 @@
 {-# LANGUAGE OverloadedLists   #-}
 {-# LANGUAGE OverloadedStrings #-}
 
-module Test.Database.LSMTree.UnitTests (tests) where
+module Test.Database.LSMTree.UnitTests (
+    tests
+    -- * Utilities
+  , ignoreBlobRef
+  ) where
 
 import           Control.Tracer (nullTracer)
 import           Data.ByteString (ByteString)