How should i mouse proof a chimney to prevent rodents from getting in?

i have rodents coming in through my chimney but don't know how to frame it, without preventing excess carbon monoxide from escaping.How should i mouse proof a chimney to prevent rodents from getting in?
http://www.ianrpubs.unl.edu/epublic/page鈥?/a>





you need to get what is called 1/2 inch hardware cloth.


What it is a metal screen made with wire and the grid is 1/2 inch squares. It is very stiff and you will not need a frame. you can just form/bend it to fit the top of the chimney. Available at most hardware stores. This is your solution ,you will see it in the first link i sent you.





http://search.yahoo.com/search?p=1/2%20i鈥?/a>How should i mouse proof a chimney to prevent rodents from getting in?
do you have screening on top of chimney. if not you can get it at places like home depot