For captive-bred Devils River minnows (Dionda diaboli), which are federally listed as threatened, we measured critical thermal maxima when they were acclimated at 18, 22, 24, and 27°C. Critical thermal maxima were 32.6, 34.3, 35.3, and 37.8°C and were positively correlated with temperatures of acclimation, but not with either gender or size. There was high pre-test mortality (39%) for fish held at 28°C for 72 days.