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We used a lethal thermal method to estimate the upper thermal tolerances of 11 native and 7 normative fish species found throughout southern Arizona. Fish were acclimated to 25 degrees C and 30 degrees C. For all species tested, an increase in acclimation temperature resulted in a higher thermal tolerance value. Among the species acclimated to 25 degrees C, desert pupfish Cyprinodon macularius, western mosquitofish Gambusia affinis, and Gila topminnow Poeciflopsis occidentalis were most tolerant to high temperature. Speckled dace Rhinichthys osculus, spikedace Meda fulgida, and loach minnow R. cobitis were least tolerant. Many native species demonstrated a limited ability to extend their upper temperature tolerances via acclimation. Our data suggest that several native species may be sensitive to increasing annual and large daily temperature fluctuations in Arizona’s streams and rivers, Although southwestern native fishes were previously believed to be tolerant to high temperature due to their evolution in desert environments, this study suggests that many of these fishes are less tolerant than previously thought. In addition, many fishes introduced from the eastern United States had higher temperature tolerances than some of the native desert species tested. Increases in stream temperatures in Arizona could reduce the habitat available for native fishes and therefore may favor those normative species with higher thermal tolerances.