With temperatures expected to rise across the southern Rocky Mountains, the ability of native fishes to tolerate stream warming has become a critical concern for those tasked with preserving coldwater species. We used common garden experiments to evaluate the thermal tolerance of cutthroat trout ( Oncorhynchus clarkii) fry from five populations important to managers representing three sub-species. Critical thermal maxima (CTMs) were evaluated through traditional exposure trials, while optimal growth and ultimate upper incipient lethal temperatures (UUILTs) were examined over the course of 21-day trials at six static temperature treatments. Whereas CTMs differed among populations (mean = 27.91 °C, SD = 0.35 °C), UUILTs did not (mean = 24.40 °C, SD = 0.04 °C). Comparison of cubic temperature-growth functions to the traditional quadratic functions showed that adding a third-order term for temperature can improve model fit, and revealed substantial differences in optimal growth temperatures (15.4–18.3 °C). Knowledge of these thermal tolerance thresholds will help to predict the consequences of a warming climate, identify suitable habitats for repatriation, and inform water quality temperature standards established to protect these fish into the future.