Some fishes, including bluegillsLepomis macrochirus, have highest critical swimming speeds at their preferred temperatures; however, it is not known why fishes select temperatures that are optimal for performance. This paper examines the relationship between selected temperatures and critical swimming speeds for bluegills to test the hypothesis that fishes select temperatures in response to available power. A rationale is presented that available power is a useful short-term measure of fitness among intraspecific competitors, that Fry's scope for activity is a measure of available power, and that critical swimming speed is a function of Fry's scope for activity and can be used as a measure of available power. Results of this study supported the hypothesis by demonstrating that for each acclimation temperature, critical swimming speeds of bluegills were highest at preferred temperatures, and decreased at other temperatures with frequency of time spent at those temperatures, although the exact nature of the relationship has yet to be determined. These results supported the concept that fishes select temperatures on the basis of available power and have implications for quantifying and predicting the energetic cost of exposure of fish to sublethal temperatures.