>>for (var i = document.myform.subcat.options.length; i >= 0; i--) >>document.myform.subcat.options[i] = null;

>When I use this (or the VBScript equivalent) I get a "not implemented" error >message! Any ideas?

I think the stuff i suggested would work fine.. ok i'll just give it in some brief.
myform is the form name. subcat is the select box which is multiple ex.
select NAME="subcat" SIZE="4" Multiple
option [List of Sub category] /option
/select

now in onchange event of another select just write a func ShowSubcategory(this)

the func definition wud contain this code
--function showsubcategory(elem)
--{
-- for (var i = document.myform.subcat.options.length; i >= 0; i--)
-- document.myform.subcat.options[i] = null;
--}
I hope it'll help