We determined prolonged swimming ability of larval and juvenile Colorado squawfish Ptychocheilus lucius at 10, 14, and 20 degree C. Fatigue velocity (velocity at which 50% of the fish failed to sustain swimming in 30-min tests) increased significantly with water temperature and fish length. Differences in swimming ability between 10 and 14 degree C became less pronounced as fish length increased, but differences remained substantial between the two cooler and the 20 degree C swimming experiments for all size-classes tested. We conclude that cold hypolimnetic releases from hydroelectric dams can significantly reduce prolonged swimming ability of young-of-year Colorado squawfish, and may help explain the decline of this species in the Colorado River basin