Inside - just at the top, so when I cinch down the closure the only thing you can get to is aluminum.- often I just put the whole pot in the top - same effect. while you probably *can* cinch the ursack opening closed enough to keep mice out, I don't trust myself to do it to keep the little buggers out - and I have had mice crawl in the hole before I learned to do that.

Note that in theory the ursack is not rodent proof - so I imagine R.O.U.S's with sharp teeth can chew through it - people sell different stuff for that, having said that, I've taken it all over the world and never had probelms with them chewing through it - and I've been in some pretty crittery spots.