diff --git a/yarn-project/foundation/src/abi/decoder.ts b/yarn-project/foundation/src/abi/decoder.ts
index 1f4b0499d28c..2d1ec6a4637d 100644
--- a/yarn-project/foundation/src/abi/decoder.ts
+++ b/yarn-project/foundation/src/abi/decoder.ts
@@ -45,9 +45,9 @@ class AbiDecoder {
       }
       case 'struct': {
         if (isU128Struct(abiType)) {
-          const hi = this.decodeNext({ kind: 'field' }) as bigint;
           const lo = this.decodeNext({ kind: 'field' }) as bigint;
-          return U128.fromU64sBE(hi, lo).toInteger();
+          const hi = this.decodeNext({ kind: 'field' }) as bigint;
+          return U128.fromU64sLE(lo, hi).toInteger();
         }
 
         const struct: { [key: string]: AbiDecoded } = {};
diff --git a/yarn-project/foundation/src/abi/encoder.ts b/yarn-project/foundation/src/abi/encoder.ts
index b3fdb81c41a0..6173663f6226 100644
--- a/yarn-project/foundation/src/abi/encoder.ts
+++ b/yarn-project/foundation/src/abi/encoder.ts
@@ -107,12 +107,11 @@ class ArgumentEncoder {
           break;
         }
         if (isU128Struct(abiType)) {
-          // U128 struct has high and low limbs - so we first convert the value to the 2 limbs and then we encode them
-          // --> this results in the limbs being added to the final flat array as [..., hi, lo, ...] (we stick to big
-          // endian!).
+          // U128 struct has low and high limbs - so we first convert the value to the 2 limbs and then we encode them
+          // --> this results in the limbs being added to the final flat array as [..., lo, hi, ...].
           const value = new U128(arg);
-          this.encodeArgument({ kind: 'field' }, value.hi, `${name}.hi`);
           this.encodeArgument({ kind: 'field' }, value.lo, `${name}.lo`);
+          this.encodeArgument({ kind: 'field' }, value.hi, `${name}.hi`);
           break;
         }
         if (isWrappedFieldStruct(abiType)) {
diff --git a/yarn-project/foundation/src/abi/u128.test.ts b/yarn-project/foundation/src/abi/u128.test.ts
index fb76bdb97298..0c18baba722e 100644
--- a/yarn-project/foundation/src/abi/u128.test.ts
+++ b/yarn-project/foundation/src/abi/u128.test.ts
@@ -29,11 +29,11 @@ describe('U128', () => {
     });
   });
 
-  describe('fromU64sBE', () => {
+  describe('fromU64sLE', () => {
     it('correctly combines valid limbs', () => {
-      const hi = 0xcafebaben;
       const lo = 0xdeadbeefn;
-      const combined = U128.fromU64sBE(hi, lo);
+      const hi = 0xcafebaben;
+      const combined = U128.fromU64sLE(lo, hi);
 
       expect(combined.lo).toBe(lo);
       expect(combined.hi).toBe(hi);
@@ -42,41 +42,41 @@ describe('U128', () => {
 
     it('accepts maximum valid limb values', () => {
       const maxLimb = 2n ** 64n - 1n;
-      const value = U128.fromU64sBE(maxLimb, maxLimb);
+      const value = U128.fromU64sLE(maxLimb, maxLimb);
 
-      expect(value.hi).toBe(maxLimb);
       expect(value.lo).toBe(maxLimb);
+      expect(value.hi).toBe(maxLimb);
       expect(value.toInteger()).toBe(2n ** 128n - 1n);
     });
 
     it('throws for invalid lower limb', () => {
       const invalid = 2n ** 64n;
-      expect(() => U128.fromU64sBE(0n, invalid)).toThrow(`Lower limb ${invalid} is not within valid range`);
+      expect(() => U128.fromU64sLE(invalid, 0n)).toThrow(`Lower limb ${invalid} is not within valid range`);
 
-      expect(() => U128.fromU64sBE(0n, -1n)).toThrow('Lower limb -1 is not within valid range');
+      expect(() => U128.fromU64sLE(-1n, 0n)).toThrow('Lower limb -1 is not within valid range');
     });
 
     it('throws for invalid higher limb', () => {
       const invalid = 2n ** 64n;
-      expect(() => U128.fromU64sBE(invalid, 0n)).toThrow(`Higher limb ${invalid} is not within valid range`);
+      expect(() => U128.fromU64sLE(0n, invalid)).toThrow(`Higher limb ${invalid} is not within valid range`);
 
-      expect(() => U128.fromU64sBE(-1n, 0n)).toThrow('Higher limb -1 is not within valid range');
+      expect(() => U128.fromU64sLE(0n, -1n)).toThrow('Higher limb -1 is not within valid range');
     });
   });
 
   describe('getters', () => {
-    it('correctly extracts hi and lo components', () => {
+    it('correctly extracts lo and hi components', () => {
       const testCases = [
-        { hi: 0xcafebaben, lo: 0xdeadbeefn },
-        { hi: 1n, lo: 0n },
-        { hi: 0n, lo: 1n },
-        { hi: 2n ** 64n - 1n, lo: 2n ** 64n - 1n },
+        { lo: 0xdeadbeefn, hi: 0xcafebaben },
+        { lo: 0n, hi: 1n },
+        { lo: 1n, hi: 0n },
+        { lo: 2n ** 64n - 1n, hi: 2n ** 64n - 1n },
       ];
 
-      for (const { hi, lo } of testCases) {
-        const value = U128.fromU64sBE(hi, lo);
-        expect(value.hi).toBe(hi);
+      for (const { lo, hi } of testCases) {
+        const value = U128.fromU64sLE(lo, hi);
         expect(value.lo).toBe(lo);
+        expect(value.hi).toBe(hi);
       }
     });
   });
diff --git a/yarn-project/foundation/src/abi/u128.ts b/yarn-project/foundation/src/abi/u128.ts
index 25a34079e413..4a0431bac074 100644
--- a/yarn-project/foundation/src/abi/u128.ts
+++ b/yarn-project/foundation/src/abi/u128.ts
@@ -15,14 +15,14 @@ export class U128 {
     this.value = value;
   }
 
-  static fromU64sBE(hi: bigint, lo: bigint): U128 {
+  static fromU64sLE(lo: bigint, hi: bigint): U128 {
     // Validate limbs are within valid ranges
-    if (hi < 0n || hi >= 2n ** 64n) {
-      throw new Error(`Higher limb ${hi} is not within valid range (0 to 2^64-1)`);
-    }
     if (lo < 0n || lo >= 2n ** 64n) {
       throw new Error(`Lower limb ${lo} is not within valid range (0 to 2^64-1)`);
     }
+    if (hi < 0n || hi >= 2n ** 64n) {
+      throw new Error(`Higher limb ${hi} is not within valid range (0 to 2^64-1)`);
+    }
 
     // Combine limbs into full value and create new instance
     const value = (hi << 64n) | lo;