diff --git a/ChangeLog.md b/ChangeLog.md
--- a/ChangeLog.md
+++ b/ChangeLog.md
@@ -1,3 +1,7 @@
+## 0.2.1.2
+
+* `Deque` optimizations by avoiding modulus operations completely.
+
 ## 0.2.1.1
 
 * Fix a bug in `Deque`'s new vector allocation/copy code.
diff --git a/Data/Mutable/Deque.hs b/Data/Mutable/Deque.hs
--- a/Data/Mutable/Deque.hs
+++ b/Data/Mutable/Deque.hs
@@ -10,6 +10,7 @@
     , module Data.Mutable.Class
     ) where
 
+import           Control.Exception            (assert)
 import           Control.Monad                (liftM)
 import           Data.Mutable.Class
 import qualified Data.Vector.Generic.Mutable  as V
@@ -75,8 +76,12 @@
         if size == 0
             then return Nothing
             else do
-                x <- V.unsafeRead v (start `mod` V.length v)
-                writeRef var $! DequeState v (start + 1) (size - 1)
+                x <- V.unsafeRead v start
+                let start' = start + 1
+                    start''
+                        | start' >= V.length v = 0
+                        | otherwise = start'
+                writeRef var $! DequeState v start'' (size - 1)
                 return $! Just x
     {-# INLINE popFront #-}
 instance V.MVector v a => MutablePopBack (Deque v s a) where
@@ -86,8 +91,11 @@
             then return Nothing
             else do
                 let size' = size - 1
-                    end = (start + size') `mod` V.length v
-                x <- V.unsafeRead v end
+                    end = start + size'
+                    end'
+                        | end >= V.length v = end - V.length v
+                        | otherwise = end
+                x <- V.unsafeRead v end'
                 writeRef var $! DequeState v start size'
                 return $! Just x
     {-# INLINE popBack #-}
@@ -101,9 +109,12 @@
                 then newVector v start size inner
                 else do
                     let size' = size + 1
-                        start' = (start - 1) `mod` V.length v
-                    V.unsafeWrite v start' x
-                    writeRef var $! DequeState v start' size'
+                        start' = (start - 1) `rem` V.length v
+                        start''
+                            | start' < 0 = V.length v + start'
+                            | otherwise = start'
+                    V.unsafeWrite v start'' x
+                    writeRef var $! DequeState v start'' size'
     {-# INLINE pushFront #-}
 instance V.MVector v a => MutablePushBack (Deque v s a) where
     pushBack (Deque var) x = do
@@ -114,8 +125,11 @@
             if size >= V.length v
                 then newVector v start size inner
                 else do
-                    let end = (start + size) `mod` V.length v
-                    V.unsafeWrite v end x
+                    let end = start + size
+                        end'
+                            | end >= V.length v = end - V.length v
+                            | otherwise = end
+                    V.unsafeWrite v end' x
                     writeRef var $! DequeState v start (size + 1)
     {-# INLINE pushBack #-}
 
@@ -125,15 +139,14 @@
           -> Int
           -> (v (PrimState m) a -> Int -> Int -> m b)
           -> m b
-newVector v start size f = do
+newVector v size2 sizeOrig f = assert (sizeOrig == V.length v) $ do
     v' <- V.unsafeNew (V.length v * 2)
     let size1 = V.length v - size2
-        size2 = start `mod` V.length v
     V.unsafeCopy
         (V.unsafeTake size1 v')
         (V.unsafeSlice size2 size1 v)
     V.unsafeCopy
         (V.unsafeSlice size1 size2 v')
         (V.unsafeTake size2 v)
-    f v' 0 size
+    f v' 0 sizeOrig
 {-# INLINE newVector #-}
diff --git a/mutable-containers.cabal b/mutable-containers.cabal
--- a/mutable-containers.cabal
+++ b/mutable-containers.cabal
@@ -1,5 +1,5 @@
 name:                mutable-containers
-version:             0.2.1.1
+version:             0.2.1.2
 synopsis:            Abstactions and concrete implementations of mutable containers
 description:         See docs and README at <http://www.stackage.org/package/mutable-containers>
 homepage:            https://github.com/fpco/mutable-containers