diff --git a/lib/fetch/index.js b/lib/fetch/index.js
index 0b99d546380..4eafb2fee2d 100644
--- a/lib/fetch/index.js
+++ b/lib/fetch/index.js
@@ -36,7 +36,8 @@ const {
   isAborted,
   isErrorLike,
   fullyReadBody,
-  readableStreamClose
+  readableStreamClose,
+  isomorphicEncode
 } = require('./util')
 const { kState, kHeaders, kGuard, kRealm } = require('./symbols')
 const assert = require('assert')
@@ -832,7 +833,7 @@ async function schemeFetch (fetchParams) {
       const body = bodyWithType[0]
 
       // 5. Let length be body’s length, serialized and isomorphic encoded.
-      const length = `${body.length}`
+      const length = isomorphicEncode(`${body.length}`)
 
       // 6. Let type be bodyWithType’s type if it is non-null; otherwise the empty byte sequence.
       const type = bodyWithType[1] ?? ''
@@ -1303,8 +1304,7 @@ async function httpNetworkOrCacheFetch (
   //    7. If contentLength is non-null, then set contentLengthHeaderValue to
   //    contentLength, serialized and isomorphic encoded.
   if (contentLength != null) {
-    // TODO: isomorphic encoded
-    contentLengthHeaderValue = String(contentLength)
+    contentLengthHeaderValue = isomorphicEncode(`${contentLength}`)
   }
 
   //    8. If contentLengthHeaderValue is non-null, then append
@@ -1327,8 +1327,7 @@ async function httpNetworkOrCacheFetch (
   //    `Referer`/httpRequest’s referrer, serialized and isomorphic encoded,
   //     to httpRequest’s header list.
   if (httpRequest.referrer instanceof URL) {
-    // TODO: isomorphic encoded
-    httpRequest.headersList.append('referer', httpRequest.referrer.href)
+    httpRequest.headersList.append('referer', isomorphicEncode(httpRequest.referrer.href))
   }
 
   //    12. Append a request `Origin` header for httpRequest.
diff --git a/lib/fetch/response.js b/lib/fetch/response.js
index f36e8406ad4..6ef9aa1c423 100644
--- a/lib/fetch/response.js
+++ b/lib/fetch/response.js
@@ -10,7 +10,8 @@ const {
   isAborted,
   isBlobLike,
   serializeJavascriptValueToJSONString,
-  isErrorLike
+  isErrorLike,
+  isomorphicEncode
 } = require('./util')
 const {
   redirectStatus,
@@ -124,8 +125,7 @@ class Response {
     responseObject[kState].status = status
 
     // 6. Let value be parsedURL, serialized and isomorphic encoded.
-    // TODO: isomorphic encoded?
-    const value = parsedURL.toString()
+    const value = isomorphicEncode(URLSerializer(parsedURL))
 
     // 7. Append `Location`/value to responseObject’s response’s header list.
     responseObject[kState].headersList.append('location', value)
diff --git a/lib/fetch/util.js b/lib/fetch/util.js
index c71ab16eec1..0ebc9d560c4 100644
--- a/lib/fetch/util.js
+++ b/lib/fetch/util.js
@@ -870,6 +870,22 @@ function readableStreamClose (controller) {
   }
 }
 
+/**
+ * @see https://infra.spec.whatwg.org/#isomorphic-encode
+ * @param {string} input
+ */
+function isomorphicEncode (input) {
+  // 1. Assert: input contains no code points greater than U+00FF.
+  for (let i = 0; i < input.length; i++) {
+    assert(input.charCodeAt(i) <= 0xFF)
+  }
+
+  // 2. Return a byte sequence whose length is equal to input’s code
+  //    point length and whose bytes have the same values as the
+  //    values of input’s code points, in the same order
+  return input
+}
+
 /**
  * Fetch supports node >= 16.8.0, but Object.hasOwn was added in v16.9.0.
  */
@@ -912,5 +928,6 @@ module.exports = {
   fullyReadBody,
   bytesMatch,
   isReadableStreamLike,
-  readableStreamClose
+  readableStreamClose,
+  isomorphicEncode
 }