A stamina tunnel was used to determine the prolonged swimming performance of age-0 humpback chubs Gila cypha, bonytail chubs G. elegans, and Colorado squawfish Ptychocheilus lucius and of subadult Colorado squawfish. The “fatigue velocity” in body lengths per second at which 50% of the test fish were fatigued (FV50) was determined at 14, 20, and 26°C. The ranges of FV50 values for the three fishes (average total length in parentheses) were: Humpback chubs (95 mm), 4.4–5.7; bonytail chubs (99 mm), 4.7–5.8; small Colorado squawfish (104 mm), 4.0–4.5; large Colorado squawfish (432 mm), 2.0–2.3. Absolute speed of large Colorado squawfish was about 2.4 times that of small Colorado squawfish. Swimming ability of the subyearlings increased with increased water temperature. These rare fish had prolonged-swimming abilities similar to other fish species.