Investigations of seasonal changes in the hematological parameters of Colossoma macropomum revealed that during times of the year when they are exposed to low oxygen concentrations (< 0.5 mg 1−1), there is an increase in the hemoglobin content of the blood and the erythrocyte count. Experiments to determine the influence of oxygen content on the routine oxygen consumption showed that the critical concentration is 2 mg 1−1 at 30°C. At concentrations below 0.5 mg O2 1−1, surface water is used for aquatic respiration. Above the critical concentration, short-term fluctuations in oxygen availability are compensated for by adjustment in the ventilation rate. Gas exchange is facilitated by the unusually large gill surface area (349 mm2g−1 for a 200 g fish).