The tolerance to high temperature and low oxygen concentration was measured for the unarmored stickleback. These fish have a critical thermal maximum (CTM) of 30.5 °C when acclimated to 8°C and a CTM of 34.6''C when acclimated to 22.7°C. Respiratory experiments indicated that the critical oxygen concentration for this species is about 2.0 ppm. Above this concentration, the respiratory rate is independent of oxygen concentration; however below 2.0 ppm, oxygen consumption rapidly declines. To maintain respiratory independence as oxygen concentration decreases, ventilatory rate rises rapidly. In an oxygen concentration of 7.4 ppm the mean ventilatory rate is 104.6 per minute. As the oxygen decreases, ventilatory rate increases to 230.7 per minute in 1.32 ppm of oxygen. In lower oxygen concentrations, gill ventilation rapidly decreases.