<select name=bbb onchange="viewaaa()">  
<option value="">请选择 </option>
<option value="1">1</option>
<option value="2">2</option>
</select>  
<div id=aaa style="display:none;">fffffffffffffff</div>

当选择1时，显示<div id=aaa style="display:none;">fffffffffffffff</div>
选择其它是不显示.

<script language="javascript">
function viewaaa(val)
{
   var divObj=document.getElementById("aaa");//定位Div
   //判断是否选择了1
   if(val=="1")
      divObj.style.display="";
   else
      divObj.style.display="none";
}
</script>
<!--this.options[this.selectedIndex].value获得下拉列表选中的值-->
<select name=bbb onchange="viewaaa(this.options[this.selectedIndex].value);">  
<option value="">请选择 </option>
<option value="1">1</option>
<option value="2">2</option>
</select>  
<div id=aaa style="display:none;">fffffffffffffff</div>

多谢版主