The effect of ambient oxygen on random swimming activity in the juveniIes of Sea bass, Lates calcarifer was found out in sea water (30 ppt), brackish water (15 ppt) and freshwater (0 ppt) at 30°C and 35°C. The random swimming activity of this species first increased and then declined at both 30°C and 35OC in sea water and brackish water on exposure to low ambient oxygen whereas at both the temperatures in freshwater, the activity exhibited a reverse trend with a rise in between. Mean rates of random swimming activity at 35°C seemed to be higher than the rates at 30°C in all the three media.