Oxygen consumption and carbon dioxide production increased with increase in random (spontaneous) swimming activity in mullet,Rhinomugil corsula in high oxygen (normoxia), but the oxygen consumption in low oxygen (hypoxia) was negatively correlated with swimming activity in tests at both 30 and 35°C. The RQ of mullet under hypoxia always remained over unity and increased with increase in activity, clearly indicating intense utilization of anaerobic energy. The RQ under normoxia remained near unity irrespective of random swimming activity change. The AQ of normoxic and hypoxic mullet showed opposing trends, the quotient increasing with decreasing activity in the former and increasing with activity increase in the latter. The AQ change suggests increased protein utilization in quieter fish when adequate ambient oxygen is available, but the hypoxic mullet utilizes more protein the more active it is. The latter change in AQ is consonant with the change in hypoxic RQ and it is likely that increased anaerobic energy utilization demanded by the increased activity is accompanied by increased protein breakdown and ammonia release, thereby helping in acid-base balance and ionic (Na+) regulation. Results obtained at 30 and 35°C are almost identical. Within the small range temperature does not seem to cause a marked metabolic difference in mullet.