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The mechanical power output of fast-twitch fibres from the iliofibularis of the lizard Dipsosaurus dorsalis was measured over a broad body temperature range using the oscillatory work-loop technique. The optimal cycling frequency, that frequency at which mechanical power output is maximal, increases with temperature from 3.3 Hz at 15-degrees-C to 20.1 Hz at 42-degrees-C. Maximum power output increases with temperature, from 20 W kg-1 at 15-degrees-C to 154 W kg-1 at 42-degrees-C, the largest power output yet measured using the work-loop technique. At low temperatures (15-degrees-C and 22-degrees-C), stride frequency during burst running is nearly identical to the optimal cycling frequency for in vitro power output, suggesting that maximum power output may limit hindlimb cycle frequency in vivo. However, at higher temperatures (35-degrees-C and 42-degrees-C), the optimal cycling frequency of the isolated muscle is significantly higher than the burst stride frequency, demonstrating that contractile events no longer limit hindlimb cycle frequency. At higher temperatures, it is thus unlikely that the fast-twitch fibres of this muscle in vivo attain their potential for maximum power output.