If your tires are 26 by a decimal number such as 26x2.1 or 26x1.9, any 26 x decimal size will fit. They are all ISO 559 which should be the size of your rim if it is a recent mountain bike (excluding 29er's). Thus, the 26x1.25 should fit your rim. If, on the other hand, your rim is 26 by a fraction, such as 26x1 1/2, then the 26 tires that are followed by a decimal number will not fit. I swapped out my 26x2.1 knobby tires for the 26x1.25 city slick, and they work great for pavement, even cracked sidewalks. They are on sale at Performance this week for $8.99 in the store. They do look skinny on my wheels compared to the 2.1 knobbys, but who cares. They roll great. The other issue to consider is getting a smaller tube, one made for 26x1 to 26x1.5 tires. It will fit a little better in the 26x1.25. I did not do that. I just used the tubes that were in my 26x2.1, removed most of the air, and the tubes fit in the 26x1.25 just fine.