diff --git includes/ajax.inc includes/ajax.inc
index 7eab334..084f59c 100644
--- includes/ajax.inc
+++ includes/ajax.inc
@@ -200,6 +200,7 @@ function ajax_get_form() {
  */
 function ajax_form_callback() {
   list($form, $form_state, $form_id, $form_build_id) = ajax_get_form();
+  $triggering_element_name = $_POST['triggering_element'];
 
   // Build, validate and if possible, submit the form.
   drupal_process_form($form_id, $form, $form_state);
@@ -208,9 +209,17 @@ function ajax_form_callback() {
   // drupal_process_form() set up.
   $form = drupal_rebuild_form($form_id, $form_state, $form_build_id);
 
-  // Get the callback function from the clicked button.
-  $ajax = $form_state['clicked_button']['#ajax'];
-  $callback = $ajax['callback'];
+  // Get the callback function from the clicked button. Processing a triggering element name
+  // like myfieldset[myfieldset2[myfielname]] or just myfieldname.
+  $triggering_element = $form;
+  foreach (preg_split('/[\[\]]+/',$triggering_element_name) as $element) {
+    if (!empty($triggering_element[$element])) {
+      $triggering_element = $triggering_element[$element];
+    }
+  }
+  if (!empty($triggering_element['#ajax']['callback'])) {
+    $callback = $triggering_element['#ajax']['callback'];
+  }
   if (drupal_function_exists($callback)) {
     $html = $callback($form, $form_state);
 
diff --git misc/ajax.js misc/ajax.js
index 0804992..4d710d0 100644
--- misc/ajax.js
+++ misc/ajax.js
@@ -142,7 +142,12 @@ Drupal.ajax = function (base, element, element_settings) {
 
   // Bind the ajaxSubmit function to the element event.
   $(this.element).bind(element_settings.event, function () {
+    // Note that if #tree is FALSE in the form, and there are triggering elements of the
+    // same name, AJAX all falls apart. Use #tree = TRUE in a form with fieldsets.
+    triggering_element = element.name;
     if (ajax.form) {
+      ajax.form.append('<input type="hidden" name="triggering_element" value="' + triggering_element + '" />');
+
       // If setClick is set, we must set this to ensure that the button's
       // value is passed.
       if (ajax.setClick) {