The physiology and behaviour of fish are strongly affected by ambient water temperature. Physiological traits related to metabolism, such as aerobic scope (AS), can be measured across temperature gradients, and the resulting performance curve reflects the thermal niche that fish can occupy. We measured AS of westslope cutthroat trout (Oncorhynchus clarkii lewisi) at 5,10,15, 20, and 22degreesC and compared temperature preference (T.sub.pref) of the species with non-native brook trout (Salvelinusfontinalis), brown trout (Salmo trutta), and rainbow trout (Oncorhynchus mykiss). Intermittent-flow respirometry experiments demonstrated that metabolic performance of westslope cutthroat trout was optimal at ~15degreesC and decreased substantially beyond this temperature, until lethal temperatures at ~25degreesC. Adjusted T.sub.pref across species were comparatively high, ranging from 17.8 to 19.9degreesC, with the highest T.sub.pref observed for westslope cutthroat trout. Results suggest that although westslope cutthroat trout is considered a cold-water species, they do not prefer or perform as well in cold water (less than or equal to10degreesC) and thus can occupy a warmer thermal niche than previously thought. The metabolic performance curve (AS) can be used to develop species-specific thermal criteria to delineate important thermal habitats and guide conservation and recovery actions for westslope cutthroat trout.