Great advise above. That time of year I'd concentrate on the lake fishing. A few hours south of Missoula is Clark Canyon Reservior. Home of huge hawgs (not to be confused with those disgusting Dawgs). It is the headwaters for the Beaverhead River which would alsobe a good bet (few miles south of Dillon). Along I-90 the Anaconda Settling ponds kick out some monster fish. As was stated in an earlier post, Tthe Blackfoot reservation has some phenomenal pond and lake fishing for big trout.

But if you are staying around Missoula don't miss the entertainment at Freds!!!! Also, get Steve Probasco's book on Montana. It has great recent info on the waters mainly in Western Mont. Geg Thomas' Montana Fly Fishing Guide is also a good bet.