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Increasing stream temperatures have important implications for arid-region fishes. Little is known about effects of high water temperatures that fluctuate over extended periods on Apache Trout Oncorhynchus gilae apache, a federally threatened species of southwestern USA streams. We compared survival and growth of juvenile Apache Trout held for 30 d in static temperatures (16, 19, 22, 25, and 28 degrees C) and fluctuating diel temperatures (+/- 3 degrees C from 16, 19, 22 and 25 degrees C midpoints and +/- 6 degrees C from 19 degrees C and 22 degrees C midpoints). Lethal temperature for 50% (LT50) of the Apache Trout under static temperatures (mean [SD] = 22.8 [0.6]degrees C) was similar to that of +/- 3 degrees C diel temperature fluctuations (23.1 [0.1]degrees C). Mean LT50 for the midpoint of the +/- 6 degrees C fluctuations could not be calculated because survival in the two treatments (19 +/- 6 degrees C and 22 +/- 6 degrees C) was not below 50%; however, it probably was also between 22 degrees C and 25 degrees C because the upper limb of a +/- 6 degrees C fluctuation on a 25 degrees C midpoint is above critical thermal maximum for Apache Trout (28.5-30.4 degrees C). Growth decreased as temperatures approached the LT50. Apache Trout can survive short-term exposure to water temperatures with daily maxima that remain below 25 degrees C and midpoint diel temperatures below 22 degrees C. However, median summer stream temperatures must remain below 19 degrees C for best growth and even lower if daily fluctuations are high (>= 12 degrees C).