diff --git a/src/bigints.nim b/src/bigints.nim
index 9ef5abb..41a504d 100644
--- a/src/bigints.nim
+++ b/src/bigints.nim
@@ -1198,3 +1198,49 @@ func powmod*(base, exponent, modulus: BigInt): BigInt =
         result = (result * basePow) mod modulus
       basePow = (basePow * basePow) mod modulus
       exponent = exponent shr 1
+
+func getLimbAndBitPosFromBitPos(bit: Natural): tuple[bit, limb: Natural] {.inline.} =
+  (bit: Natural(bit and 31), limb: Natural(bit shr 5))
+
+func setBit*(a: var BigInt; bit: Natural) =
+  ## Mutates `a`, with the bit at position `bit` set to 1.
+  runnableExamples:
+    var v = 0b0000_0011.initBigInt
+    v.setBit(5)
+    doAssert v == 0b0010_0011.initBigInt
+
+  let (b, l) = getLimbAndBitPosFromBitPos(bit)
+
+  if l >= a.limbs.len:
+    a.limbs.setLen(l + 1)
+
+  a.limbs[l] = a.limbs[l] or (1'u32 shl b)
+
+func clearBit*(a: var BigInt; bit: Natural) =
+  ## Mutates `v`, with the bit at position `bit` set to 0.
+  runnableExamples:
+     var v = 0b0000_0011.initBigInt
+     v.clearBit(1)
+     doAssert v == 0b0000_0001.initBigInt
+
+  let (b, l) = getLimbAndBitPosFromBitPos(bit)
+
+  if l >= a.limbs.len:
+    return
+
+  a.limbs[l] = a.limbs[l] and not (1'u32 shl b)
+  normalize(a)
+
+func testBit*(a: BigInt; bit: Natural): bool =
+  ## Returns true if the bit in `a` at positions `bit` is set to 1.
+  runnableExamples:
+    let v = 0b0000_1111.initBigInt
+    doAssert v.testBit(0)
+    doAssert not v.testBit(7)
+
+  let (b, l) = getLimbAndBitPosFromBitPos(bit)
+
+  if l >= a.limbs.len:
+    false
+  else:
+    (a.limbs[l] and (1'u32 shl b)) != 0
diff --git a/tests/tbigints.nim b/tests/tbigints.nim
index 3b3a59d..95f6073 100644
--- a/tests/tbigints.nim
+++ b/tests/tbigints.nim
@@ -786,6 +786,88 @@ proc main() =
     doAssert pred(a, 3) == initBigInt(4)
     doAssert succ(a, 3) == initBigInt(10)
 
+  block: # setBit/clearBit/clearBit
+    var a = initBigInt(0)
+    for i in 0..256:
+      doAssert not a.testBit(i)
+    for i in 0..256:
+      a.setBit(i)
+      for j in 0..i:
+        doAssert a.testBit(j)
+      for j in (i + 1)..256:
+        doAssert not a.testBit(j)
+    for i in countDown(256, 0):
+      doAssert a.testBit(i)
+    for i in countDown(256, 0):
+      a.clearBit(i)
+      for j in 0..(i - 1):
+        doAssert a.testBit(j)
+      for j in i..256:
+        doAssert not a.testBit(j)
+    for i in countDown(256, 0):
+      doAssert not a.testBit(i)
+    for i in 1..256:
+      a.setBit(i)
+      doAssert a.testBit(i)
+      doAssert not a.testBit(i - 1)
+      doAssert not a.testBit(i + 1)
+      a.clearBit(i)
+      doAssert not a.testBit(i)
+      doAssert not a.testBit(i - 1)
+      doAssert not a.testBit(i + 1)
+      doAssert a == initBigInt(0)
+
+    a = initBigInt(0)
+    a.setBit(0)
+    doAssert a == initBigInt(1)
+    doAssert a.testBit(0)
+    a.setBit(1)
+    doAssert a == initBigInt(3)
+    doAssert a.testBit(0) and a.testBit(1)
+    a.setBit(31)
+    doAssert a == initBigInt((1 shl 31) + 3)
+    doAssert a.testBit(0) and a.testBit(1) and a.testBit(31)
+    a.setBit(30)
+    doAssert a == initBigInt((1 shl 31) + (1 shl 30) + 3)
+    doAssert a.testBit(0) and a.testBit(1) and a.testBit(30) and a.testBit(31)
+    a.clearBit(31)
+    doAssert a == initBigInt((1 shl 30) + 3)
+    doAssert a.testBit(0) and a.testBit(1) and a.testBit(30) and (not a.testBit(31))
+    a.clearBit(30)
+    doAssert a == initBigInt(3)
+    doAssert a.testBit(0) and a.testBit(1) and (not a.testBit(30)) and (not a.testBit(31))
+    a.clearBit(1)
+    doAssert a == initBigInt(1)
+    doAssert a.testBit(0) and (not a.testBit(1)) and (not a.testBit(30)) and (not a.testBit(31))
+    a.clearBit(0)
+    doAssert a == initBigInt(0)
+
+    a = initBigInt(0)
+    a.setBit(63)
+    doAssert a == initBigInt(1'u64 shl 63'u64)
+    doAssert (not a.testBit(1)) and (not a.testBit(31)) and (not a.testBit(32)) and a.testBit(63)
+    a.setBit(1)
+    doAssert a == initBigInt((1'u64 shl 63'u64) + 2)
+    doAssert a.testBit(1) and (not a.testBit(31)) and (not a.testBit(32)) and a.testBit(63)
+    a.setBit(32)
+    doAssert a == initBigInt((1'u64 shl 63'u64) + (1'u64 shl 32'u64) + 2)
+    doAssert a.testBit(1) and (not a.testBit(31)) and a.testBit(32) and a.testBit(63)
+    a.setBit(31)
+    doAssert a == initBigInt((1'u64 shl 63'u64) + (1'u64 shl 32'u64) + (1'u64 shl 31'u64) + 2)
+    doAssert a.testBit(1) and a.testBit(31) and a.testBit(32) and a.testBit(63)
+
+    a.clearBit(63)
+    doAssert a == initBigInt((1'u64 shl 32'u64) + (1'u64 shl 31'u64) + 2)
+    doAssert a.testBit(1) and a.testBit(31) and a.testBit(32) and (not a.testBit(63))
+    a.clearBit(1)
+    doAssert a == initBigInt((1'u64 shl 32'u64) + (1'u64 shl 31'u64))
+    doAssert (not a.testBit(1)) and a.testBit(31) and a.testBit(32) and (not a.testBit(63))
+    a.clearBit(32)
+    doAssert a == initBigInt((1'u64 shl 31'u64))
+    doAssert (not a.testBit(1)) and a.testBit(31) and (not a.testBit(32)) and (not a.testBit(63))
+    a.clearBit(31)
+    doAssert a == initBigInt(0)
+    doAssert (not a.testBit(1)) and (not a.testBit(31)) and (not a.testBit(32)) and (not a.testBit(63))
 
 static: main()
 main()