diff --git a/emcc.py b/emcc.py
index 313d68b4dd0f..d3e8230cc76e 100755
--- a/emcc.py
+++ b/emcc.py
@@ -2846,14 +2846,19 @@ def modularize():
       # after document.currentScript is gone, so we save it.
       # (when MODULARIZE_INSTANCE, an instance is created
       # immediately anyhow, like in non-modularize mode)
+      if shared.Settings.EXPORT_ES6:
+        script_url = "import.meta.url"
+      else:
+        script_url = "typeof document !== 'undefined' && document.currentScript ? document.currentScript.src : undefined"
       src = '''
 var %(EXPORT_NAME)s = (function() {
-  var _scriptDir = typeof document !== 'undefined' && document.currentScript ? document.currentScript.src : undefined;
+  var _scriptDir = %(script_url)s;
   return (%(src)s);
 })();
 ''' % {
         'EXPORT_NAME': shared.Settings.EXPORT_NAME,
-        'src': src
+        'src': src,
+        'script_url': script_url
       }
   else:
     # Create the MODULARIZE_INSTANCE instance
diff --git a/emscripten.py b/emscripten.py
index d415557bdb7e..e511850c7d09 100644
--- a/emscripten.py
+++ b/emscripten.py
@@ -1699,6 +1699,11 @@ def create_receiving(function_table_data, function_tables_defs, exported_impleme
 
   shared.Settings.MODULE_EXPORTS = module_exports = exported_implemented_functions + function_tables(function_table_data)
 
+  # Currently USE PTHREADS + EXPORT ES6 doesn't work with the previously generated assertions.
+  # ES6 modules disallow re-assigning read-only properties.
+  if shared.Settings.USE_PTHREADS and shared.Settings.EXPORT_ES6 and shared.Settings.ASSERTIONS and receiving:
+    receiving = "assert(!ENVIRONMENT_IS_PTHREAD, 'Error: USE_PTHREADS and EXPORT_ES6 requires ASSERTIONS=0');\n" + receiving
+
   if not shared.Settings.SWAPPABLE_ASM_MODULE:
     if shared.Settings.DECLARE_ASM_MODULE_EXPORTS:
       imported_exports = [s for s in module_exports if s not in initializers]
diff --git a/src/shell_pthreads.js b/src/shell_pthreads.js
index 6b1aeaa17053..e4f9802aa9de 100644
--- a/src/shell_pthreads.js
+++ b/src/shell_pthreads.js
@@ -4,10 +4,9 @@
 // 3) We could be an application pthread running in a worker. (ENVIRONMENT_IS_WORKER == true and ENVIRONMENT_IS_PTHREAD == true)
 #if USE_PTHREADS
 
-if (typeof ENVIRONMENT_IS_PTHREAD === 'undefined') {
-  // ENVIRONMENT_IS_PTHREAD=true will have been preset in worker.js. Make it false in the main runtime thread.
-  // N.B. this line needs to appear without 'var' keyword to avoid 'var hoisting' from occurring. (https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/var)
-  ENVIRONMENT_IS_PTHREAD = false;
+// ENVIRONMENT_IS_PTHREAD=true will have been preset in worker.js. Make it false in the main runtime thread.
+var ENVIRONMENT_IS_PTHREAD = Module.ENVIRONMENT_IS_PTHREAD || false;
+if (!ENVIRONMENT_IS_PTHREAD) {
   var PthreadWorkerInit = {}; // Collects together variables that are needed at initialization time for the web workers that host pthreads.
 }
 #if MODULARIZE
diff --git a/src/worker.js b/src/worker.js
index 44afa4ccc734..9205872591a8 100644
--- a/src/worker.js
+++ b/src/worker.js
@@ -23,7 +23,6 @@ var buffer; // All pthreads share the same Emscripten HEAP as SharedArrayBuffer
 var DYNAMICTOP_PTR = 0;
 var DYNAMIC_BASE = 0;
 
-var ENVIRONMENT_IS_PTHREAD = true;
 var PthreadWorkerInit = {};
 
 // performance.now() is specced to return a wallclock time in msecs since that Web Worker/main thread launched. However for pthreads this can cause
@@ -35,6 +34,10 @@ var __performance_now_clock_drift = 0;
 // Cannot use console.log or console.error in a web worker, since that would risk a browser deadlock! https://bugzilla.mozilla.org/show_bug.cgi?id=1049091
 // Therefore implement custom logging facility for threads running in a worker, which queue the messages to main thread to print.
 var Module = {};
+#if EXPORT_ES6
+var PThread;
+var HEAPU32;
+#endif
 
 #if ASSERTIONS
 function assert(condition, text) {
@@ -128,7 +131,19 @@ this.onmessage = function(e) {
 #endif
 
       {{{ makeAsmExportAndGlobalAssignTargetInPthread('PthreadWorkerInit') }}} = e.data.PthreadWorkerInit;
+      Module['ENVIRONMENT_IS_PTHREAD'] = true;
 
+#if MODULARIZE && EXPORT_ES6
+      import(e.data.urlOrBlob).then(function({{{ EXPORT_NAME }}}) {
+        Module = {{{ EXPORT_NAME }}}.default(Module);
+        PThread = Module['PThread'];
+        HEAPU32 = Module['HEAPU32'];
+#if !ASMFS
+        if (typeof FS !== 'undefined' && typeof FS.createStandardStreams === 'function') FS.createStandardStreams();
+#endif
+        postMessage({ cmd: 'loaded' });
+      });
+#else
       if (typeof e.data.urlOrBlob === 'string') {
         importScripts(e.data.urlOrBlob);
       } else {
@@ -136,7 +151,6 @@ this.onmessage = function(e) {
         importScripts(objectUrl);
         URL.revokeObjectURL(objectUrl);
       }
-
 #if MODULARIZE
 #if !MODULARIZE_INSTANCE
       Module = {{{ EXPORT_NAME }}}(Module);
@@ -149,6 +163,7 @@ this.onmessage = function(e) {
       if (typeof FS !== 'undefined' && typeof FS.createStandardStreams === 'function') FS.createStandardStreams();
 #endif
       postMessage({ cmd: 'loaded' });
+#endif
     } else if (e.data.cmd === 'objectTransfer') {
       PThread.receiveObjectTransfer(e.data);
     } else if (e.data.cmd === 'run') { // This worker was idle, and now should start executing its pthread entry point.