$(document).ready(function() {
    $('#propertycat').change(function(){
        sid = $(this).val();
        datastring="sid="+sid;
        $.ajax({
            type:"POST",
            url:"ajax.php?act=propertycat",
            data: datastring,
            success: function(data){
                $('#proptype').html(data);
            }
        });
    });
    
});