How to apply theme_menu_tree to only the top  menu
In order to theme the top level ul tag only:

function THEMENAME_menu_tree__menu_top_menu(&amp;$variables) {
 return '&lt;ul class="menu myclass"&gt;' . $variables['tree'] . '&lt;/ul&gt;';
}


You have to put this piece of code in your custom theme's template.php file and replace THEMENAME with your theme's name.