Urbanization alters stream and watershed hydrology so that fish from urban stream systems are confronted with extreme flows during storms and runoff events. To test whether residence in urban streams is associated with altered swimming ability, we compared sprint and endurance swimming performances of eight populations of blacknose dace (Rhinichthys atratulus) from different watersheds along an urban/rural gradient. Watershed impervious surface cover, a measure of urbanization, was significantly correlated with sprint performance in dace from all stream types and endurance swimming performance (U(crit)) when only fish from urban streams were analyzed. Three estimators of water flow in a stream system, watershed area, mean annual discharge, and base-flow current speed, were all related to U(crit) in fish from nonurban streams. The U(crit) was significantly repeatable after 6 mo in the laboratory, but dace populations with exceptional U(crit) values lost ability under no-flow, "detraining" conditions. Sprint performance changed substantially in individual dace after 10 wk under no-flow conditions and was a significant function of the animal's original performance. Animals with high sprint performance tended to lose ability, whereas those with poor performance gained ability. Interpopulation differences in both sprint and endurance swimming were robust over multiple years of collection from the same sites.