Размышление на тему ...
<select id="sel" ></select>
<script language="JavaScript" type="text/javascript">
var responseText='<option>-----</option><option value="1">ххх</option><option value="2">ууу</option>';
var a=document.getElementById('sel');
a.outerHTML='<select id="sel" >'+responseText+'</select>'
a.innerHTML=responseText
</script>