diff --git a/Setup.hs b/Setup.hs
deleted file mode 100644
--- a/Setup.hs
+++ /dev/null
@@ -1,2 +0,0 @@
-import Distribution.Simple
-main = defaultMain
diff --git a/changelog.md b/changelog.md
--- a/changelog.md
+++ b/changelog.md
@@ -1,3 +1,7 @@
+# 0.3.2.0
+
+* Migrate from `integer-gmp` to `ghc-bignum`.
+
 # 0.3.1.0
 
 * Performance improvement for decimal digits.
diff --git a/fast-digits.cabal b/fast-digits.cabal
--- a/fast-digits.cabal
+++ b/fast-digits.cabal
@@ -1,5 +1,5 @@
 name: fast-digits
-version: 0.3.1.0
+version: 0.3.2.0
 license: GPL-3
 license-file: LICENSE
 maintainer: andrew.lelechenko@gmail.com
@@ -14,11 +14,11 @@
 category: Data
 build-type: Simple
 cabal-version: 2.0
-extra-source-files:
+extra-doc-files:
   changelog.md
   README.md
 tested-with:
-  GHC ==7.10.3 GHC ==8.0.2 GHC ==8.2.2 GHC ==8.4.4 GHC ==8.6.5 GHC ==8.8.4 GHC ==8.10.4 GHC ==9.0.1
+  GHC ==9.0.2 GHC ==9.2.8 GHC ==9.4.5 GHC ==9.6.2
 
 source-repository head
   type: git
@@ -29,12 +29,10 @@
     Data.FastDigits
   hs-source-dirs: src
   default-language: Haskell2010
-  ghc-options: -Wall -O2 -fno-warn-deprecations
-  if impl(ghc >= 8.0)
-    ghc-options: -Wcompat
+  ghc-options: -Wall -O2 -Wcompat
   build-depends:
-    base >=4.8 && <5,
-    integer-gmp >=1.0,
+    base >=4.15 && <5,
+    ghc-bignum <1.4,
     fast-digits-internal
 
 library fast-digits-internal
@@ -42,9 +40,7 @@
     Data.FastDigits.Internal
   hs-source-dirs: src-internal
   default-language: Haskell2010
-  ghc-options: -Wall -O2
-  if impl(ghc >= 8.0)
-    ghc-options: -Wcompat
+  ghc-options: -Wall -O2 -Wcompat
   build-depends:
     base >=4.8 && <5
 
@@ -53,16 +49,14 @@
   main-is: Tests.hs
   hs-source-dirs: tests
   default-language: Haskell2010
-  ghc-options: -Wall
-  if impl(ghc >= 8.0)
-    ghc-options: -Wcompat
+  ghc-options: -Wall -Wcompat
   build-depends:
     base,
-    tasty,
-    tasty-quickcheck,
-    tasty-smallcheck,
-    QuickCheck,
-    smallcheck,
+    tasty <1.5,
+    tasty-quickcheck <0.11,
+    tasty-smallcheck <0.9,
+    QuickCheck <2.15,
+    smallcheck <1.3,
     -- digits,
     fast-digits,
     fast-digits-internal
@@ -72,11 +66,9 @@
   main-is: Bench.hs
   hs-source-dirs: bench
   default-language: Haskell2010
-  ghc-options: -Wall -O2
-  if impl(ghc >= 8.0)
-    ghc-options: -Wcompat
+  ghc-options: -Wall -O2 -Wcompat
   build-depends:
     base,
     -- digits,
     fast-digits,
-    tasty-bench >= 0.2.4
+    tasty-bench >= 0.2.4 && <0.4
diff --git a/src-internal/Data/FastDigits/Internal.hs b/src-internal/Data/FastDigits/Internal.hs
--- a/src-internal/Data/FastDigits/Internal.hs
+++ b/src-internal/Data/FastDigits/Internal.hs
@@ -7,6 +7,7 @@
 -}
 
 {-# LANGUAGE BangPatterns  #-}
+{-# LANGUAGE CPP           #-}
 {-# LANGUAGE MagicHash     #-}
 {-# LANGUAGE UnboxedTuples #-}
 
@@ -32,7 +33,7 @@
   where
     go pw = case timesWord2# pw pw of
         (# 0##, pw2 #)
-          -> let (# n, pw2n #) = go pw2 in
+          -> let !(# n, pw2n #) = go pw2 in
             case timesWord2# pw pw2n of
               (# 0##, pw2n1 #) -> (#n `timesWord#` 2## `plusWord#` 1##, pw2n1 #)
               _ -> (# n `timesWord#` 2##, pw2n #)
diff --git a/src/Data/FastDigits.hs b/src/Data/FastDigits.hs
--- a/src/Data/FastDigits.hs
+++ b/src/Data/FastDigits.hs
@@ -23,16 +23,16 @@
 
 import Data.Bits (finiteBitSize)
 import GHC.Exts (Word#, Word(..), uncheckedShiftRL#, and#, timesWord2#, minusWord#, quotRemWord#, timesWord#, Int(..), iShiftRL#, isTrue#, word2Int#, (>#), (*#))
-import GHC.Integer.GMP.Internals (GmpLimb#, BigNat, quotRemBigNatWord, isZeroBigNat, sizeofBigNat#)
 import Data.FastDigits.Internal (selectPower)
 import GHC.Natural (Natural(..))
+import GHC.Num.BigNat (BigNat(..), bigNatIsZero, bigNatQuotRemWord#, bigNatSize#, BigNat#)
 
-digitsNatural :: GmpLimb# -> BigNat -> [Word]
+digitsNatural :: Word# -> BigNat# -> [Word]
 digitsNatural base = f
   where
     f n
-      | isZeroBigNat n = []
-      | otherwise      = let !(# q, r #) = n `quotRemBigNatWord` base in
+      | bigNatIsZero n = []
+      | otherwise      = let !(# q, r #) = n `bigNatQuotRemWord#` base in
                          W# r : f q
 
 digitsWord :: Word# -> Word# -> [Word]
@@ -91,12 +91,12 @@
 
 -- | For a given base, power and precalculated base^power
 --   take an integer and return the list of its digits.
-digitsNatural' :: Word# -> Word# -> Word# -> BigNat -> [Word]
+digitsNatural' :: Word# -> Word# -> Word# -> BigNat# -> [Word]
 digitsNatural' base power poweredBase = f
   where
-    f :: BigNat -> [Word]
-    f n = let !(# q, r #) = n `quotRemBigNatWord` poweredBase in
-      if isZeroBigNat q
+    f :: BigNat# -> [Word]
+    f n = let !(# q, r #) = n `bigNatQuotRemWord#` poweredBase in
+      if bigNatIsZero q
         then digitsWord base r
         else let !(# fr, lr #) = digitsWordL base power r in
           fr ++ replicate (I# (word2Int# lr)) 0 ++ f q
@@ -111,16 +111,16 @@
   -> Natural
   -> [Word]
 digitsUnsigned (W# base) (NatS# n) = digitsWord base n
-digitsUnsigned (W# base) (NatJ# n)
-  | halfSize <- sizeofBigNat# n `iShiftRL#` 1#
+digitsUnsigned (W# base) (NatJ# n@(BN# n#))
+  | halfSize <- bigNatSize# n# `iShiftRL#` 1#
   , isTrue# (halfSize ># 128#)
   = let pow = I# (word2Int# power *# halfSize) in
     let (nHi, nLo) = NatJ# n `quotRem` (NatS# poweredBase ^ (I# halfSize)) in
     padUpTo pow (digitsUnsigned (W# base) nLo) ++ digitsUnsigned (W# base) nHi
   | otherwise
   = case power of
-    1## -> digitsNatural base n
-    _   -> digitsNatural' base power poweredBase n
+    1## -> digitsNatural base n#
+    _   -> digitsNatural' base power poweredBase n#
   where
     !(# power, poweredBase #) = selectPower base