Ajax drop down list problem with callback
The values will be passed to the callback function reload instead of the original form function, therefore you should rebuild the form in your callback function - reload and pass back the element:

function reload($form, &amp;$form_state) {
  $form_state['rebuild'] = TRUE;
  $form = drupal_rebuild_form('mymodule_form', $form_state, $form);
  return $form['select_str'];
}