you can build muscle without getting fat but most likely you wll be shortchanging your gains by trying to stay so lean. In many cases you wont gain any muscle at all by trying to keep a visible 6-pack. You need extra calories to build muscle and it's damn difficult if not impossible to eat only the exact amount of extra needed to keep your gains optimal but with no added fat.
on the other hand I dont think there is any reason a person needs to become a Pillsbury doughboy while bulking. The trick is to keep fat gains under control while maximising your muscular gains and that will simply take some experimentation with your diet.

here is a link to a site that explains it all pretty well:

http://www.geocities.com/elitemadcow1/Topics/Diet.htm

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