<select name="select" size="1" onchange="USER.value=value">
<option value=aaa selected>使用登录名</option>
<option value=bbb>输入新用户名</option>
</select>
<option value=aaa selected>使用登录名</option>
<option value=bbb>输入新用户名</option>
</select>
<script>
function ww(a)
{
f1.t1.value=a}
</script></head><body>
<form name="f1">
<SELECT onchange=ww(options[this.selectedIndex].value)>
<option value=aa>使用登录名</option>
<option value=bb>输入新用户名</option></select>
<input type="text" name="t1">
</form>
</body></html>