The present study was designed to study of effects of the leached effluent of Alumina industry (NALCO) situated at Damonjodi, Koraput on a fresh water fish, Oreochromis mossambicus, Peters (Tilapia) and its ecological significance. The effluent of the industry was tested and found to be deadly toxic. The effluent along with the red mud wastes generated from the industry was dumped in the Red mud pond for storage and for natural evaporation. Fishes were exposed to graded series of concentrations of the effluent for acute toxicity studies. The MAC value deduced was 3.15% of effluent in 50 liters of water for 30th days. A safety concentration of 3.1% of effluent l-50 was selected for this study. The LC10 , LC50 , LC90 and LC100 , determined for Tilapia fish was found to be 3.45, 8.15, 12.6 and 15.5%. l-50 respectively after 30th days of exposure. Exposed fishes appeared lethargic when compared to the control fish. In appetence and ataxia was observed in the exposed fish after first day of introduction of the effluent. Effluent exposed fishes showed erratic movements, loss of equilibrium, gradual onset of inactivity when compared to the control fish. The body weight of the effluent exposed fish decreased significantly, when compared to control fish, where a significant increase in body weight was marked. In the exposed fish 53.2% decrease in body weight was marked on 28d exposure. When the exposed fish was transferred to toxicant free medium, partial recovery was noted. The body weight depleted by 51.7% during recovery period after 28th days. A partial recovery by 1.5% was noted. The BSI and HSI significantly declined in the leached effluent exposed fish when compared to the control fish. A maximum of 21.8% decrease was noted in BSI of leached effluent exposed fish, where partial recovery by 9.5% was noted after 28th days of recovery. The LSI decreased by 38.4% after 28th days of exposure and partial recovery by 10.5% was noted after 28th days of recovery when the exposed fish was transferred to toxicant free medium. The exposed fish could not recover to its pre-test activity even after 56th days of recovery double the exposure period indicating damage to vital systems.