diff --git a/runtime/src/esm_http_test.rs b/runtime/src/esm_http_test.rs
index 5fd9ac8..4580d1c 100644
--- a/runtime/src/esm_http_test.rs
+++ b/runtime/src/esm_http_test.rs
@@ -248,6 +248,24 @@ fn http_import_failures_reject_with_the_cause() {
     assert_eq!(invalid, "TypeError");
 }
 
+#[test]
+fn worker_entry_served_over_http() {
+    let server = TestServer::start();
+    let o = server.origin.clone();
+    server.route("/ns/m/echo.worker.js", "text/javascript", "import { add } from './add.js';\nself.onmessage = (event) => self.postMessage(add(event.data));");
+    server.route("/ns/m/add.js", "text/javascript", "export const add = (n) => n + 1;");
+    let mut runtime = Runtime::new(".");
+    runtime.register_delegate_isolate_ptr();
+    let run = |path: &str| {
+        format!(
+            "new Promise((resolve, reject) => {{ const worker = new Worker(new URL('{o}{path}')); worker.onmessage = (event) => {{ worker.terminate(); event.type === 'messageerror' ? reject(event.error) : resolve(event.data); }}; worker.postMessage(41); }})"
+        )
+    };
+    assert_eq!(await_js(&mut runtime, &run("/ns/m/echo.worker.js")), "ok:42");
+    let missing = await_js(&mut runtime, &run("/ns/m/missing.worker.js"));
+    assert!(missing.starts_with("err:") && missing.contains("status=404"), "{missing}");
+}
+
 #[test]
 fn native_websocket_round_trip() {
     let server = TestServer::start();
diff --git a/runtime/src/worker_support.rs b/runtime/src/worker_support.rs
index da678d1..21065e0 100644
--- a/runtime/src/worker_support.rs
+++ b/runtime/src/worker_support.rs
@@ -26,6 +26,12 @@ pub fn install_worker_runtime(scope: &mut ContextScope<HandleScope>) {
                 return { source: input, filename: '[worker:eval]' };
             }
 
+            // A dev server's URL (`new URL('./x.worker.ts', import.meta.url)` under HMR) isn't a
+            // file: the worker's thread imports it, so its own imports resolve against the URL.
+            if (/^https?:\/\//.test(input)) {
+                return { source: '', filename: input };
+            }
+
             if (typeof globalThis.__nsResolveModulePath !== 'function' || typeof globalThis.__nsReadTextFile !== 'function') {
                 throw new Error('Worker file loading is unavailable: missing runtime host hooks');
             }
@@ -90,6 +96,10 @@ pub fn install_worker_runtime(scope: &mut ContextScope<HandleScope>) {
                 typeof globalThis.__nsWorkerPollMessagesBlocking === 'function' &&
                 typeof globalThis.__nsWorkerTerminate === 'function';
 
+            if (!canUseThreadedHost && /^https?:\/\//.test(resolved.filename)) {
+                throw new Error('Loading a Worker from a URL needs the threaded worker host: ' + resolved.filename);
+            }
+
             var channel    = new MessageChannel();
             var mainPort   = channel.port1;
             var workerPort = channel.port2;
diff --git a/runtime/src/worker_threads.rs b/runtime/src/worker_threads.rs
index 8b536bc..ae28770 100644
--- a/runtime/src/worker_threads.rs
+++ b/runtime/src/worker_threads.rs
@@ -173,7 +173,11 @@ fn worker_bootstrap_script(source: &str, filename: &str) -> Result<String, Strin
                     }});
                 }};
 
-                if (typeof globalThis.__nsEvalAsModule === 'function') {{
+                if (/^https?:\/\//.test(__workerFilename)) {{
+                    import(__workerFilename).catch(function (error) {{
+                        __queueMessage({{ __workerError: String((error && error.message) || error) }});
+                    }});
+                }} else if (typeof globalThis.__nsEvalAsModule === 'function') {{
                     globalThis.__nsEvalAsModule(__workerSource, __workerFilename || '[worker]');
                 }} else {{
                     const exec = new Function('__filename', __workerSource);