diff --git a/.changeset/exact-event-stream-type.md b/.changeset/exact-event-stream-type.md
new file mode 100644
index 0000000..b2c4fee
--- /dev/null
+++ b/.changeset/exact-event-stream-type.md
@@ -0,0 +1,5 @@
+---
+'eventsource': patch
+---
+
+Accept case variations of `text/event-stream` and reject different MIME subtypes such as `text/event-streaming`.
diff --git a/src/EventSource.ts b/src/EventSource.ts
index e4083b4..75ed50f 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 (!/^[\t\n\r ]*text\/event-stream[\t\n\r ]*(?:;|$)/i.test(contentType)) {
       this.#failConnection('Invalid content type, expected "text/event-stream"', status)
       return
     }
diff --git a/test/client.test.ts b/test/client.test.ts
index e26a891..546cada 100644
--- a/test/client.test.ts
+++ b/test/client.test.ts
@@ -856,6 +856,38 @@ browserTest(
   },
 )
 
+test.each([
+  {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: ' \ttext/event-stream\t ; charset=iso-8859-1', valid: true},
+  {contentType: 'text/event-stream; note="a,b"', valid: true},
+  {contentType: 'text/event-stream;', valid: true},
+  {contentType: 'text/event-streaming', valid: false},
+  {contentType: 'text/event-stream+json', valid: false},
+  {contentType: 'text/event-stream/extra', 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},
+])('validates the event stream MIME type: $contentType', async ({contentType, valid}) => {
+  const onResult = getCallCounter<Event>({name: 'MIME result'})
+  const es = new OurEventSource(
+    `${serverUrl}/?${new URLSearchParams({'content-type': contentType})}`,
+    esInit,
+  )
+  es.addEventListener('welcome', onResult.listener)
+  es.addEventListener('error', onResult.listener)
+
+  try {
+    await onResult.waitForCallCount(1)
+    expect(onResult.lastArg.type).toBe(valid ? 'welcome' : 'error')
+    expect(es.readyState).toBe(valid ? OurEventSource.OPEN : OurEventSource.CLOSED)
+  } 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 a8e0646..9798e2c 100644
--- a/test/helpers/server.ts
+++ b/test/helpers/server.ts
@@ -100,9 +100,12 @@ export function handleRequest(
   }
 }
 
-function writeDefault(_req: IncomingMessage, res: ServerResponse) {
+function writeDefault(req: IncomingMessage, res: ServerResponse) {
+  const contentType =
+    new URL(req.url || '/', 'http://localhost').searchParams.get('content-type') ??
+    'text/event-stream'
   res.writeHead(200, {
-    'Content-Type': 'text/event-stream',
+    'Content-Type': contentType,
     'Cache-Control': 'no-cache',
     Connection: 'keep-alive',
   })