diff --git a/test/FeeGrowthDirection.t.sol b/test/FeeGrowthDirection.t.sol
new file mode 100644
index 000000000..e42c9868c
--- /dev/null
+++ b/test/FeeGrowthDirection.t.sol
@@ -0,0 +1,182 @@
+// SPDX-License-Identifier: UNLICENSED
+pragma solidity ^0.8.20;
+
+import {Test} from "forge-std/Test.sol";
+import {Deployers} from "./utils/Deployers.sol";
+import {IHooks} from "../src/interfaces/IHooks.sol";
+import {IPoolManager} from "../src/interfaces/IPoolManager.sol";
+import {PoolKey} from "../src/types/PoolKey.sol";
+import {PoolId} from "../src/types/PoolId.sol";
+import {ModifyLiquidityParams} from "../src/types/PoolOperation.sol";
+import {Currency, CurrencyLibrary} from "../src/types/Currency.sol";
+import {StateLibrary} from "../src/libraries/StateLibrary.sol";
+
+/// @notice Asserts fee growth globals update on the correct token index after swaps.
+/// @dev Closes https://github.com/Uniswap/v4-core/issues/606
+contract FeeGrowthDirectionTest is Test, Deployers {
+    using StateLibrary for IPoolManager;
+
+    function setUp() public {
+        deployFreshManagerAndRouters();
+        deployMintAndApprove2Currencies();
+    }
+
+    function test_feeGrowthDirection_zeroForOne_exactInput() public {
+        (PoolKey memory poolKey,) =
+            initPoolAndAddLiquidity(currency0, currency1, IHooks(address(0)), 3000, SQRT_PRICE_1_1);
+        PoolId poolId = poolKey.toId();
+
+        (uint256 fg0, uint256 fg1) = manager.getFeeGrowthGlobals(poolId);
+        assertEq(fg0, 0);
+        assertEq(fg1, 0);
+
+        swap(poolKey, true, -1 ether, ZERO_BYTES);
+
+        (fg0, fg1) = manager.getFeeGrowthGlobals(poolId);
+        assertGt(fg0, 0, "feeGrowthGlobal0 must increase for zeroForOne swap");
+        assertEq(fg1, 0, "feeGrowthGlobal1 must remain 0 for zeroForOne swap");
+    }
+
+    function test_feeGrowthDirection_oneForZero_exactInput() public {
+        (PoolKey memory poolKey,) =
+            initPoolAndAddLiquidity(currency0, currency1, IHooks(address(0)), 3000, SQRT_PRICE_1_1);
+        PoolId poolId = poolKey.toId();
+
+        swap(poolKey, false, -1 ether, ZERO_BYTES);
+
+        (uint256 fg0, uint256 fg1) = manager.getFeeGrowthGlobals(poolId);
+        assertGt(fg1, 0, "feeGrowthGlobal1 must increase for oneForZero swap");
+        assertEq(fg0, 0, "feeGrowthGlobal0 must remain 0 for oneForZero swap");
+    }
+
+    function test_feeGrowthDirection_zeroForOne_exactOutput() public {
+        (PoolKey memory poolKey,) =
+            initPoolAndAddLiquidity(currency0, currency1, IHooks(address(0)), 500, SQRT_PRICE_1_1);
+        PoolId poolId = poolKey.toId();
+
+        swap(poolKey, true, -0.1 ether, ZERO_BYTES);
+
+        (uint256 fg0, uint256 fg1) = manager.getFeeGrowthGlobals(poolId);
+        assertGt(fg0, 0, "feeGrowthGlobal0 must increase for zeroForOne exact-output");
+        assertEq(fg1, 0, "feeGrowthGlobal1 must remain 0 for zeroForOne exact-output");
+    }
+
+    function test_feeGrowthDirection_oneForZero_exactOutput() public {
+        (PoolKey memory poolKey,) =
+            initPoolAndAddLiquidity(currency0, currency1, IHooks(address(0)), 500, SQRT_PRICE_1_1);
+        PoolId poolId = poolKey.toId();
+
+        swap(poolKey, false, -0.1 ether, ZERO_BYTES);
+
+        (uint256 fg0, uint256 fg1) = manager.getFeeGrowthGlobals(poolId);
+        assertGt(fg1, 0, "feeGrowthGlobal1 must increase for oneForZero exact-output");
+        assertEq(fg0, 0, "feeGrowthGlobal0 must remain 0 for oneForZero exact-output");
+    }
+
+    function test_feeGrowthDirection_bidirectional_independent() public {
+        (PoolKey memory poolKey,) =
+            initPoolAndAddLiquidity(currency0, currency1, IHooks(address(0)), 3000, SQRT_PRICE_1_1);
+        PoolId poolId = poolKey.toId();
+
+        swap(poolKey, true, -1 ether, ZERO_BYTES);
+
+        (uint256 fg0Mid, uint256 fg1Mid) = manager.getFeeGrowthGlobals(poolId);
+        assertGt(fg0Mid, 0);
+        assertEq(fg1Mid, 0);
+
+        swap(poolKey, false, -1 ether, ZERO_BYTES);
+
+        (uint256 fg0Final, uint256 fg1Final) = manager.getFeeGrowthGlobals(poolId);
+        assertEq(fg0Final, fg0Mid, "feeGrowthGlobal0 unchanged after oneForZero swap");
+        assertGt(fg1Final, 0, "feeGrowthGlobal1 must increase after oneForZero swap");
+    }
+
+    function test_feeGrowthDirection_withTickCrossing_zeroForOne() public {
+        (PoolKey memory poolKey,) = initPool(currency0, currency1, IHooks(address(0)), 3000, SQRT_PRICE_1_1);
+        PoolId poolId = poolKey.toId();
+
+        modifyLiquidityRouter.modifyLiquidity(
+            poolKey,
+            ModifyLiquidityParams({tickLower: -60, tickUpper: 60, liquidityDelta: 10 ether, salt: 0}),
+            ZERO_BYTES
+        );
+        modifyLiquidityRouter.modifyLiquidity(
+            poolKey,
+            ModifyLiquidityParams({tickLower: -120, tickUpper: 120, liquidityDelta: 10 ether, salt: 0}),
+            ZERO_BYTES
+        );
+
+        swap(poolKey, true, -5 ether, ZERO_BYTES);
+
+        (uint256 fg0, uint256 fg1) = manager.getFeeGrowthGlobals(poolId);
+        assertGt(fg0, 0, "feeGrowthGlobal0 must increase after tick-crossing zeroForOne");
+        assertEq(fg1, 0, "feeGrowthGlobal1 must remain 0 after tick-crossing zeroForOne");
+    }
+
+    function test_feeGrowthDirection_withTickCrossing_oneForZero() public {
+        (PoolKey memory poolKey,) = initPool(currency0, currency1, IHooks(address(0)), 3000, SQRT_PRICE_1_1);
+        PoolId poolId = poolKey.toId();
+
+        modifyLiquidityRouter.modifyLiquidity(
+            poolKey,
+            ModifyLiquidityParams({tickLower: -60, tickUpper: 60, liquidityDelta: 10 ether, salt: 0}),
+            ZERO_BYTES
+        );
+        modifyLiquidityRouter.modifyLiquidity(
+            poolKey,
+            ModifyLiquidityParams({tickLower: -120, tickUpper: 120, liquidityDelta: 10 ether, salt: 0}),
+            ZERO_BYTES
+        );
+
+        swap(poolKey, false, -5 ether, ZERO_BYTES);
+
+        (uint256 fg0, uint256 fg1) = manager.getFeeGrowthGlobals(poolId);
+        assertEq(fg0, 0, "feeGrowthGlobal0 must remain 0 after tick-crossing oneForZero");
+        assertGt(fg1, 0, "feeGrowthGlobal1 must increase after tick-crossing oneForZero");
+    }
+
+    function test_fuzz_feeGrowthDirection(bool zeroForOne, uint256 amountSeed) public {
+        uint256 swapAmount = bound(amountSeed, 1e15, 1 ether);
+        (PoolKey memory poolKey,) =
+            initPoolAndAddLiquidity(currency0, currency1, IHooks(address(0)), 3000, SQRT_PRICE_1_1);
+        PoolId poolId = poolKey.toId();
+
+        swap(poolKey, zeroForOne, -int256(swapAmount), ZERO_BYTES);
+
+        (uint256 fg0, uint256 fg1) = manager.getFeeGrowthGlobals(poolId);
+
+        if (zeroForOne) {
+            assertGt(fg0, 0, "fuzz: feeGrowthGlobal0 should increase for zeroForOne");
+            assertEq(fg1, 0, "fuzz: feeGrowthGlobal1 should remain 0 for zeroForOne");
+        } else {
+            assertEq(fg0, 0, "fuzz: feeGrowthGlobal0 should remain 0 for oneForZero");
+            assertGt(fg1, 0, "fuzz: feeGrowthGlobal1 should increase for oneForZero");
+        }
+    }
+
+    function test_feeGrowthDirection_native_zeroForOne() public {
+        (PoolKey memory poolKey,) = initPoolAndAddLiquidityETH(
+            CurrencyLibrary.ADDRESS_ZERO, currency1, IHooks(address(0)), 3000, SQRT_PRICE_1_1, 10 ether
+        );
+        PoolId poolId = poolKey.toId();
+
+        swapNativeInput(poolKey, true, -1 ether, ZERO_BYTES, 1 ether);
+
+        (uint256 fg0, uint256 fg1) = manager.getFeeGrowthGlobals(poolId);
+        assertGt(fg0, 0, "native: feeGrowthGlobal0 must increase for zeroForOne");
+        assertEq(fg1, 0, "native: feeGrowthGlobal1 must remain 0 for zeroForOne");
+    }
+
+    function test_feeGrowthDirection_native_oneForZero() public {
+        (PoolKey memory poolKey,) = initPoolAndAddLiquidityETH(
+            CurrencyLibrary.ADDRESS_ZERO, currency1, IHooks(address(0)), 3000, SQRT_PRICE_1_1, 10 ether
+        );
+        PoolId poolId = poolKey.toId();
+
+        swap(poolKey, false, -1 ether, ZERO_BYTES);
+
+        (uint256 fg0, uint256 fg1) = manager.getFeeGrowthGlobals(poolId);
+        assertEq(fg0, 0, "native: feeGrowthGlobal0 must remain 0 for oneForZero");
+        assertGt(fg1, 0, "native: feeGrowthGlobal1 must increase for oneForZero");
+    }
+}