diff --git a/lib/bcoin/protocol/constants.js b/lib/bcoin/protocol/constants.js
index 7bd97895..73bae982 100644
--- a/lib/bcoin/protocol/constants.js
+++ b/lib/bcoin/protocol/constants.js
@@ -164,7 +164,8 @@ exports.opcodes = {
   nop1: 0xb0,
   // nop2: 0xb1,
   checklocktimeverify: 0xb1,
-  nop3: 0xb2,
+  // nop3: 0xb2,
+  checksequenceverify: 0xb2,
   nop4: 0xb3,
   nop5: 0xb4,
   nop6: 0xb5,
@@ -268,6 +269,10 @@ exports.hd = {
 
 exports.locktimeThreshold = 500000000; // Tue Nov 5 00:53:20 1985 UTC
 
+exports.sequenceLocktimeDisableFlag = 0x80000000; // (1 << 31)
+exports.sequenceLocktimeTypeFlag = 1 << 22;
+exports.sequenceLocktimeMask = 0x0000ffff;
+
 exports.oneHash = new Buffer(
   '0000000000000000000000000000000000000000000000000000000000000001',
   'hex'
@@ -298,7 +303,9 @@ exports.flags = {
   VERIFY_CLEANSTACK: (1 << 8),
   VERIFY_CHECKLOCKTIMEVERIFY: (1 << 9),
   VERIFY_WITNESS: (1 << 10),
-  VERIFY_DISCOURAGE_UPGRADABLE_WITNESS_PROGRAM: (1 << 11)
+  VERIFY_DISCOURAGE_UPGRADABLE_WITNESS_PROGRAM: (1 << 11),
+  // NOTE: Should be (1 << 10) - but that conflicts with segwit
+  VERIFY_CHECKSEQUENCEVERIFY: (1 << 12)
 };
 
 // Block validation
@@ -316,4 +323,5 @@ exports.flags.STANDARD_VERIFY_FLAGS =
   | exports.flags.VERIFY_CHECKLOCKTIMEVERIFY
   | exports.flags.VERIFY_LOW_S;
   // | exports.flags.VERIFY_WITNESS
-  // | exports.flags.VERIFY_DISCOURAGE_UPGRADABLE_WITNESS_PROGRAM;
+  // | exports.flags.VERIFY_DISCOURAGE_UPGRADABLE_WITNESS_PROGRAM
+  // | exports.flags.VERIFY_CHECKSEQUENCEVERIFY;
diff --git a/lib/bcoin/script.js b/lib/bcoin/script.js
index 6986267e..3ebd998b 100644
--- a/lib/bcoin/script.js
+++ b/lib/bcoin/script.js
@@ -613,7 +613,6 @@ script.execute = function execute(data, stack, tx, index, flags, version, recurs
 
     switch (o) {
       case 'nop':
-      case 'nop3':
       case 'nop4':
       case 'nop5':
       case 'nop6':
@@ -1172,6 +1171,46 @@ script.execute = function execute(data, stack, tx, index, flags, version, recurs
 
         break;
       }
+      case 'checksequenceverify': {
+        // OP_CHECKSEQUENCEVERIFY = OP_NOP3
+        if (!(flags & constants.flags.VERIFY_CHECKSEQUENCEVERIFY)) {
+          if (flags & constants.flags.VERIFY_DISCOURAGE_UPGRADABLE_NOPS)
+            return false;
+          break;
+        }
+
+        if (!tx || stack.length === 0)
+          return false;
+
+        locktime = stack[stack.length - 1];
+
+        if (!Buffer.isBuffer(locktime))
+          return false;
+
+        if (locktime.length > 5) {
+          utils.debug('Warning: sequence is over 40 bits.');
+          return false;
+        }
+
+        locktime = script.num(locktime);
+
+        // if (locktime < 0)
+        //   return false;
+
+        // if ((locktime & constants.sequenceLocktimeDisableFlag) !== 0)
+        //   break;
+
+        if (locktime.cmpn(0) < 0)
+          return false;
+
+        if (locktime.uandn(constants.sequenceLocktimeDisableFlag).cmpn(0) !== 0)
+          break;
+
+        if (!script.checkSequenceBN(locktime, tx, i))
+          return false;
+
+        break;
+      }
       case 'nop1': {
         // OP_EVAL = OP_NOP1
         if (!script.allowEval) {
@@ -1221,6 +1260,66 @@ script.execute = function execute(data, stack, tx, index, flags, version, recurs
   return true;
 };
 
+script.checkSequence = function checkSequence(sequence, tx, i) {
+  var txSequence = tx.inputs[i].sequence;
+  var locktimeMask, txSequenceMasked, sequenceMasked;
+
+  if (tx.version < 2)
+    return false;
+
+  // if (txSequence & constants.sequenceLocktimeDisableFlag)
+    return false;
+
+  locktimeMask = constants.sequenceLocktimeTypeFlag
+    | constants.sequenceLocktimeMask;
+  txSequenceMasked = txSequence & locktimeMask;
+  sequenceMasked = sequence & locktimeMask;
+
+  if (!(
+      (txSequenceMasked < constants.sequenceLocktimeTypeFlag
+        && sequenceMasked < constants.sequenceLocktimeTypeFlag
+      || (txSequenceMasked >= constants.sequenceLocktimeTypeFlag
+        && sequenceMasked >= constants.sequenceLocktimeTypeFlag
+  )) {
+    return false;
+  }
+
+  if (sequenceMasked > txSequenceMasked)
+    return false;
+
+  return true;
+};
+
+script.checkSequenceBN = function checkSequence(sequence, tx, i) {
+  var txSequence = new bn(tx.inputs[i].sequence);
+  var locktimeMask, txSequenceMasked, sequenceMasked;
+
+  if (tx.version < 2)
+    return false;
+
+  if (txSequence.uandn(constants.sequenceLocktimeDisableFlag).cmpn(0) !== 0)
+    return false;
+
+  locktimeMask = new bn(constants.sequenceLocktimeTypeFlag)
+    .uorn(constants.sequenceLocktimeMask);
+  txSequenceMasked = txSequence.uand(locktimeMask);
+  sequenceMasked = sequence.uand(locktimeMask);
+
+  if (!(
+      (txSequenceMasked.cmpn(constants.sequenceLocktimeTypeFlag) < 0
+        && sequenceMasked.cmpn(constants.sequenceLocktimeTypeFlag) < 0
+      || (txSequenceMasked.cmpn(constants.sequenceLocktimeTypeFlag) >= 0
+        && sequenceMasked.cmpn(constants.sequenceLocktimeTypeFlag) >= 0
+  )) {
+    return false;
+  }
+
+  if (sequenceMasked.cmpn(txSequenceMasked) > 0)
+    return false;
+
+  return true;
+};
+
 script.bool = function bool(value) {
   var i;