You can probably fit anything from a 26 x 1.5 up to a 26 x 2.5 on it. (2.5 might be a bit of a 'squeeze' on an older MTB). If you just want a 26 x 2.0 or smaller just pick one up at your local bike shop. If you want to go bigger, such as a 26 x 2.5, take the bike into the bike shop to see if a tire that large will fit the fork and rear frame triangle.