Differences in seasonal patterns of growth, length at age, and diet were evident between brown trout (Salmo trutta) residing in geothermally heated and unheated sections of the Firehole River. Maximum temperatures at the warmest station exceeded 28 C and averaged about 10.5 C higher than the unheated station throughout the year. Scale characteristics indicated that brown trout from the heated stations had two annual growth periods while those from the unheated station had one, relatively short, annual growth period. Brown trout from the heated stations were significantly longer at any particular age than those from the unheated station. Caddis flies were the numerically dominant food in the unheated water while trout at the heated station fed most extensively on dipterans, molluscs, and mayflies. Additionally, trout in the unheated water fed almost entirely on immature benthic insects while those at the heated station fed heavily on emerging insects and exhibited considerably greater numbers of organisms per stomach. Rainbow trout (Salmo gairdneri), numerous only at the heated station, had seasonal growth pattern, length at age, and diet similar to those of brown trout at this station.