diff --git a/patterns/merkle-proofs/MerkleProofs.sol b/patterns/merkle-proofs/MerkleProofs.sol
index 85d2faf..7231ccf 100644
--- a/patterns/merkle-proofs/MerkleProofs.sol
+++ b/patterns/merkle-proofs/MerkleProofs.sol
@@ -9,12 +9,26 @@ contract MerkleDrop {
     // Whether a member has claimed their drop.
     // Note this assumes each members is only eligible for a single drop, i.e.,
     // they occur in the merkle tree only once.
-    mapping (address => bool) public hasClaimed;
+     mapping(uint256 => uint256) private claimedBitMap;
 
     constructor(bytes32 root) payable {
         ROOT = root;
     }
 
+    function isClaimed(uint256 index) public view returns (bool) {
+        uint256 claimedWordIndex = index / 256;
+        uint256 claimedBitIndex = index % 256;
+        uint256 claimedWord = claimedBitMap[claimedWordIndex];
+        uint256 mask = (1 << claimedBitIndex);
+        return claimedWord & mask == mask;
+    }
+
+    function _setClaimed(uint256 index) private {
+        uint256 claimedWordIndex = index / 256;
+        uint256 claimedBitIndex = index % 256;
+        claimedBitMap[claimedWordIndex] = claimedBitMap[claimedWordIndex] | (1 << claimedBitIndex);
+    }
+
     // Given a leaf hash in a merkle tree and a list of sibling hashes/nodes,
     // attempt to arrive at the root hash, returning true if we got there.
     function prove(bytes32 leaf, bytes32[] memory siblings) public view returns (bool) {
@@ -34,17 +48,19 @@ contract MerkleDrop {
     // Claim a drop on behalf of a member, given a proof that their claim belongs
     // to the merkle tree.
     function claim(
+        uint256 index,
         address payable member,
         uint256 claimAmount,
         bytes32[] memory proof
     )
         external
     {
+        require(!isClaimed(index), "already claimed");
         // Security note: Leaf hashes are inverted to prevent second preimage attacks,
         // i.e., passing in intermediate node values (subtree hashes) for member and
         // claimAmount.
         require(prove(~keccak256(abi.encode(member, claimAmount)), proof), 'bad proof');
-        hasClaimed[member] = true;
+        _setClaimed(index);
         member.transfer(claimAmount);
     }
 }
diff --git a/test/MerkleProofs.t.sol b/test/MerkleProofs.t.sol
index 2e4ee74..ff9a252 100644
--- a/test/MerkleProofs.t.sol
+++ b/test/MerkleProofs.t.sol
@@ -228,7 +228,7 @@ contract MerkleProofsTest is Test, TestUtils {
         uint256 memberIndex = _randomUint256() % members.length;
         bytes32[] memory proof = helper.createProof(memberIndex, tree);
         vm.prank(members[memberIndex]);
-        drop.claim(payable(members[memberIndex]), claimAmounts[memberIndex], proof);
+        drop.claim(memberIndex, payable(members[memberIndex]), claimAmounts[memberIndex], proof);
         assertEq(members[memberIndex].balance, claimAmounts[memberIndex]);
     }