Así:
Código PHP:
<?php
$query_Recordset = "SELECT tabla1.id,tabla2.id AS Id,tabla1.descrip FROM tabla1,tabla2 WHERE tabla2.llave= '".$llave."' AND tabla2.id=tabla1.id ";
$Recordset = mysql_query($query_Recordset, $conex) or die(mysql_error());
$row_Recordset = mysql_fetch_assoc($Recordset);
$totalRows_Recordset = mysql_num_rows($Recordset);
do{
?>
<option value="<?php echo $row_Recordset['id']; ?>"><?php echo $row_Recordset['descrip'];?> </option>
<?php }while($row_Recordset= mysql_fetch_assoc($Recordset)); ?>
</select>