diff --git a/.changeset/fix-content-type-essence.md b/.changeset/fix-content-type-essence.md
new file mode 100644
index 0000000..63afd5f
--- /dev/null
+++ b/.changeset/fix-content-type-essence.md
@@ -0,0 +1,5 @@
+---
+'eventsource': patch
+---
+
+Accept event stream MIME types regardless of case. Reject different subtypes and select the last valid type when Content-Type contains multiple values.
diff --git a/src/EventSource.ts b/src/EventSource.ts
index e4dd9c6..834fe0c 100644
--- a/src/EventSource.ts
+++ b/src/EventSource.ts
@@ -484,7 +484,7 @@ class EventSourceImpl extends EventTarget implements EventSource {
 
     // [spec] …or if res's `Content-Type` is not `text/event-stream`, then fail the connection.
     const contentType = headers.get('content-type') || ''
-    if (!contentType.startsWith('text/event-stream')) {
+    if (getMimeTypeEssence(contentType) !== 'text/event-stream') {
       this.#failConnection('Invalid content type, expected "text/event-stream"', status)
       return
     }
@@ -810,3 +810,38 @@ function defineEventProperty(
     configurable: true,
   })
 }
+
+// Fetch selects the last valid MIME type, ignoring invalid values and */*.
+// Commas inside quoted parameters belong to the same value.
+// https://fetch.spec.whatwg.org/#concept-header-extract-mime-type
+function getMimeTypeEssence(contentType: string): string | undefined {
+  let essence: string | undefined
+  let start = 0
+  let quoted = false
+
+  for (let i = 0; i <= contentType.length; i++) {
+    const char = contentType[i]
+    if (i < contentType.length) {
+      if (quoted && char === '\\' && i + 1 < contentType.length) {
+        i++
+        continue
+      }
+      if (char === '"') {
+        quoted = !quoted
+        continue
+      }
+      if (char !== ',' || quoted) continue
+    }
+
+    const candidate = contentType
+      .slice(start, i)
+      .match(
+        /^[\t\n\r ]*([!#$%&'*+.^_`|~0-9A-Za-z-]+\/[!#$%&'*+.^_`|~0-9A-Za-z-]+)[\t\n\r ]*(?:;|$)/,
+      )?.[1]
+      ?.toLowerCase()
+    if (candidate && candidate !== '*/*') essence = candidate
+    start = i + 1
+  }
+
+  return essence
+}
diff --git a/test/client.test.ts b/test/client.test.ts
index d807576..387363b 100644
--- a/test/client.test.ts
+++ b/test/client.test.ts
@@ -793,6 +793,89 @@ browserTest(
   },
 )
 
+test.each([
+  {contentType: 'text/event-stream', valid: true},
+  {contentType: 'Text/Event-Stream', valid: true},
+  {contentType: 'TEXT/EVENT-STREAM', valid: true},
+  {contentType: 'text/event-stream;charset=utf-8', valid: true},
+  {contentType: 'Text/Event-Stream; charset=UTF-8', valid: true},
+  {contentType: 'text/event-stream ; charset=utf-8', valid: true},
+  {contentType: ' \ttext/event-stream\t ; charset=utf-8', valid: true},
+  {contentType: 'text/event-stream; boundary="one;two"', valid: true},
+  {contentType: 'text/event-stream;', valid: true},
+  {contentType: 'text/event-stream; charset', valid: true},
+  {contentType: 'text/event-stream; charset=iso-8859-1', valid: true},
+  {contentType: 'text/plain, Text/Event-Stream; charset=utf-8', valid: true},
+  {contentType: 'text/event-stream, invalid', valid: true},
+  {contentType: 'text/event-stream, */*', valid: true},
+  {contentType: 'text/event-stream,', valid: true},
+  {contentType: ', text/event-stream', valid: true},
+  {contentType: 'text/event-stream; note="a,b"', valid: true},
+  {contentType: 'text/event-stream; note="a\\\",b"', valid: true},
+  {contentType: 'text/event-stream; note="unclosed, text/plain', valid: true},
+  {contentType: 'text/plain; note="a,b", text/event-stream', valid: true},
+  {contentType: ['text/plain', 'Text/Event-Stream'], valid: true},
+  {contentType: ['text/event-stream', 'invalid'], valid: true},
+  {contentType: ['text/event-stream', '*/*'], valid: true},
+  {contentType: null, valid: false},
+  {contentType: '', valid: false},
+  {contentType: 'text/plain', valid: false},
+  {contentType: 'text/event-streaming', valid: false},
+  {contentType: 'text/event-stream+json', valid: false},
+  {contentType: 'text/event-stream/extra', valid: false},
+  {contentType: 'text/event-stream, text/plain', valid: false},
+  {contentType: 'text/event-stream; charset=utf-8, text/plain', valid: false},
+  {contentType: 'text/plain; note="x, text/event-stream"', valid: false},
+  {contentType: 'text/event-stream; note="a\\\",b", text/plain', valid: false},
+  {contentType: ['text/event-stream', 'text/plain'], valid: false},
+  {contentType: 'text/event-stream garbage', valid: false},
+  {contentType: 'text /event-stream', valid: false},
+  {contentType: 'text/ event-stream', valid: false},
+  {contentType: 'text/event-stream\u00a0; charset=utf-8', valid: false},
+  {contentType: '\u00a0text/event-stream', valid: false},
+])('checks SSE MIME essence for $contentType', async ({contentType, valid}) => {
+  const params = new URLSearchParams()
+  if (contentType !== null) {
+    for (const value of Array.isArray(contentType) ? contentType : [contentType]) {
+      params.append('value', value)
+    }
+  }
+
+  let signal: AbortSignal | undefined
+  const onMessage = getCallCounter({name: 'MIME message'})
+  const onError = getCallCounter<ErrorEvent>({name: 'MIME error'})
+  const es = new OurEventSource(`${serverUrl}/content-type?${params}`, {
+    ...esInit,
+    fetch(url, init) {
+      signal = init.signal
+      return request(url, init)
+    },
+  })
+  es.addEventListener('message', onMessage.listener)
+  es.addEventListener('error', onError.listener)
+
+  try {
+    const outcome = await Promise.race([
+      onMessage.waitForCallCount(1).then(() => 'message'),
+      onError.waitForCallCount(1).then(() => 'error'),
+    ])
+    expect(outcome).toBe(valid ? 'message' : 'error')
+
+    if (valid) {
+      expect(es.readyState).toBe(OurEventSource.OPEN)
+      expect(onMessage.lastArg.data).toBe('Hello, world!')
+      expect(onError.callCount).toBe(0)
+    } else {
+      expect(es.readyState).toBe(OurEventSource.CLOSED)
+      expect(onError.lastArg.message).toBe('Invalid content type, expected "text/event-stream"')
+      expect(onMessage.callCount).toBe(0)
+      expect(signal?.aborted).toBe(true)
+    }
+  } finally {
+    es.close()
+  }
+})
+
 test.each([200, 403])(
   'aborts a rejected HTTP %i response before reporting failure',
   async (status) => {
diff --git a/test/helpers/server.ts b/test/helpers/server.ts
index ace2733..ee6788a 100644
--- a/test/helpers/server.ts
+++ b/test/helpers/server.ts
@@ -75,6 +75,8 @@ export function handleRequest(
       return writeDebug(req, res)
     case '/invalid-stream':
       return writeInvalidStream(req, res)
+    case '/content-type':
+      return writeContentType(req, res)
     case '/set-cookie':
       return writeCookies(req, res)
     case '/authed':
@@ -581,6 +583,13 @@ function writeInvalidStream(req: IncomingMessage, res: ServerResponse) {
   res.write('This response is not a usable event stream.')
 }
 
+function writeContentType(req: IncomingMessage, res: ServerResponse) {
+  const contentTypes = new URL(req.url || '/', 'http://localhost').searchParams.getAll('value')
+  res.writeHead(200, contentTypes.length === 0 ? {} : {'Content-Type': contentTypes})
+  tryWrite(res, encode({data: 'Hello, world!'}))
+  tryWrite(res, ':\n')
+}
+
 function writeFallback(_req: IncomingMessage, res: ServerResponse) {
   res.writeHead(404, {
     'Content-Type': 'text/plain',