diff --git a/foundry/src/uniswap_x/UniswapXFiller.sol b/foundry/src/uniswap_x/UniswapXFiller.sol
index 1156c31..7826e3c 100644
--- a/foundry/src/uniswap_x/UniswapXFiller.sol
+++ b/foundry/src/uniswap_x/UniswapXFiller.sol
@@ -9,6 +9,7 @@ import {
 } from "@openzeppelin/contracts/token/ERC20/utils/SafeERC20.sol";
 import "@openzeppelin/contracts/access/AccessControl.sol";
 import "@openzeppelin/contracts/utils/Address.sol";
+import {TychoRouter} from "../TychoRouter.sol";
 
 error UniswapXFiller__AddressZero();
 error UniswapXFiller__BatchExecutionNotSupported();
@@ -19,6 +20,7 @@ contract UniswapXFiller is AccessControl, IReactorCallback {
     // UniswapX V2DutchOrder Reactor
     IReactor public immutable reactor;
     address public immutable tychoRouter;
+    address public immutable nativeAddress;
 
     // keccak256("NAME_OF_ROLE") : save gas on deployment
     bytes32 public constant REACTOR_ROLE =
@@ -30,7 +32,11 @@ contract UniswapXFiller is AccessControl, IReactorCallback {
         address indexed token, uint256 amount, address indexed receiver
     );
 
-    constructor(address _tychoRouter, address _reactor) {
+    constructor(
+        address _tychoRouter,
+        address _reactor,
+        address _native_address
+    ) {
         if (_tychoRouter == address(0)) revert UniswapXFiller__AddressZero();
         if (_reactor == address(0)) revert UniswapXFiller__AddressZero();
 
@@ -38,6 +44,9 @@ contract UniswapXFiller is AccessControl, IReactorCallback {
         _grantRole(REACTOR_ROLE, address(_reactor));
         tychoRouter = _tychoRouter;
         reactor = IReactor(_reactor);
+
+        // slither-disable-next-line missing-zero-check
+        nativeAddress = _native_address;
     }
 
     function execute(SignedOrder calldata order, bytes calldata callbackData)
@@ -58,12 +67,20 @@ contract UniswapXFiller is AccessControl, IReactorCallback {
 
         ResolvedOrder memory order = resolvedOrders[0];
 
-        // TODO properly handle native in and out tokens
-        uint256 ethValue = 0;
+        bool tokenInApprovalNeeded = bool(uint8(callbackData[0]) == 1);
+        bool tokenOutApprovalNeeded = bool(uint8(callbackData[1]) == 1);
+        bytes calldata tychoCalldata = bytes(callbackData[2:]);
+
+        // The TychoRouter will take the input tokens from the filler
+        if (tokenInApprovalNeeded) {
+            // Native ETH input is not supported by UniswapX
+            IERC20(order.input.token).forceApprove(
+                tychoRouter, type(uint256).max
+            );
+        }
 
         // slither-disable-next-line low-level-calls
-        (bool success, bytes memory result) =
-            tychoRouter.call{value: ethValue}(callbackData);
+        (bool success, bytes memory result) = tychoRouter.call(tychoCalldata);
 
         if (!success) {
             revert(
@@ -75,10 +92,19 @@ contract UniswapXFiller is AccessControl, IReactorCallback {
             );
         }
 
-        // Multiple outputs are possible when taking fees - but token itself should
-        // not change.
-        IERC20 token = IERC20(order.outputs[0].token);
-        token.forceApprove(address(reactor), type(uint256).max);
+        if (tokenOutApprovalNeeded) {
+            // Multiple outputs are possible when taking fees - but token itself should
+            // not change.
+            OutputToken memory output = order.outputs[0];
+            if (output.token != nativeAddress) {
+                IERC20 token = IERC20(output.token);
+                token.forceApprove(address(reactor), type(uint256).max);
+            } else {
+                // With native ETH - the filler is responsible for transferring back
+                // to the reactor.
+                Address.sendValue(payable(address(reactor)), output.amount);
+            }
+        }
     }
 
     /**
diff --git a/foundry/test/uniswap_x/UniswapXFiller.t.sol b/foundry/test/uniswap_x/UniswapXFiller.t.sol
index 8102e4f..5128339 100644
--- a/foundry/test/uniswap_x/UniswapXFiller.t.sol
+++ b/foundry/test/uniswap_x/UniswapXFiller.t.sol
@@ -23,7 +23,7 @@ contract UniswapXFillerTest is Test, TychoRouterTestSetup {
 
     function fillerSetup() public {
         vm.startPrank(ADMIN);
-        filler = new UniswapXFiller(tychoRouterAddr, REACTOR);
+        filler = new UniswapXFiller(tychoRouterAddr, REACTOR, address(0));
         fillerAddr = address(filler);
         filler.grantRole(keccak256("EXECUTOR_ROLE"), EXECUTOR);
         vm.stopPrank();
@@ -31,12 +31,12 @@ contract UniswapXFillerTest is Test, TychoRouterTestSetup {
 
     function testTychoAddressZeroTychoRouter() public {
         vm.expectRevert(UniswapXFiller__AddressZero.selector);
-        filler = new UniswapXFiller(address(0), REACTOR);
+        filler = new UniswapXFiller(address(0), REACTOR, address(0));
     }
 
     function testTychoAddressZeroReactor() public {
         vm.expectRevert(UniswapXFiller__AddressZero.selector);
-        filler = new UniswapXFiller(tychoRouterAddr, address(0));
+        filler = new UniswapXFiller(tychoRouterAddr, address(0), address(0));
     }
 
     function testCallback() public {
@@ -55,7 +55,7 @@ contract UniswapXFillerTest is Test, TychoRouterTestSetup {
         bytes memory swap =
             encodeSingleSwap(address(usv2Executor), protocolData);
 
-        bytes memory callbackData = abi.encodeWithSelector(
+        bytes memory tychoRouterData = abi.encodeWithSelector(
             tychoRouter.singleSwap.selector,
             amountIn,
             WETH_ADDR,
@@ -68,6 +68,9 @@ contract UniswapXFillerTest is Test, TychoRouterTestSetup {
             swap
         );
 
+        bytes memory callbackData =
+            abi.encodePacked(true, true, tychoRouterData);
+
         deal(WETH_ADDR, address(filler), amountIn);
         vm.startPrank(address(filler));
         IERC20(WETH_ADDR).approve(tychoRouterAddr, amountIn);
@@ -76,11 +79,11 @@ contract UniswapXFillerTest is Test, TychoRouterTestSetup {
         OutputToken[] memory outputs = new OutputToken[](1);
         outputs[0] = OutputToken({
             token: address(DAI_ADDR),
-            // Irrelevant fields - we only need the token address for approval
             amount: 1847751195973566072891,
             recipient: BOB
         });
-        // All irrelevant fields for this test - we only need the output token address
+        // Mostly irrelevant fields for this test - we only need token input and outputs
+        // info for the sake of testing.
         orders[0] = ResolvedOrder({
             info: OrderInfo({
                 reactor: address(0),
@@ -90,7 +93,12 @@ contract UniswapXFillerTest is Test, TychoRouterTestSetup {
                 additionalValidationContract: address(0),
                 additionalValidationData: ""
             }),
-            input: InputToken({token: address(WETH_ADDR), amount: 0, maxAmount: 0}),
+            input: InputToken({
+                token: address(WETH_ADDR),
+                amount: amountIn,
+                // We need the proper maxAmount for our approval to work
+                maxAmount: amountIn
+            }),
             outputs: outputs,
             sig: "",
             hash: ""
@@ -133,7 +141,7 @@ contract UniswapXFillerTest is Test, TychoRouterTestSetup {
         bytes memory swap =
             encodeSingleSwap(address(usv2Executor), protocolData);
 
-        bytes memory callbackData = abi.encodeWithSelector(
+        bytes memory tychoRouterData = abi.encodeWithSelector(
             tychoRouter.singleSwap.selector,
             amountIn,
             WBTC_ADDR,
@@ -146,6 +154,12 @@ contract UniswapXFillerTest is Test, TychoRouterTestSetup {
             swap
         );
 
+        bytes memory callbackData = abi.encodePacked(
+            true, // tokenIn approval needed
+            true, // tokenOut approval needed
+            tychoRouterData
+        );
+
         vm.startPrank(address(filler));
         IERC20(WBTC_ADDR).approve(tychoRouterAddr, amountIn);
         vm.stopPrank();