Copper and lead are among the most widely common pollutants distributed in the aquatic environment. The 96-hr LC50 bioassays were carried out to determine the toxicity of these two heavy metals (cu &pb) on the blue tilapia: Oreochromis aureus. The results showed that the 96-hr LCse of copper and lead were 8.17 and 25.86 mg/L, respec­tively .Groups of adult and healthy 0.aureus were subjected to the acute (lethal) concentrattion (96-LC50)of each metal and examined alter 24 and 96 hours. Other groups were subjected to the chronic (sublethal) concentration (1/10LC50) of each metal and examined after 6 and 12 weeks. The lethal and sublethal concentrations of copper and lead al­tered the behavioural activities of the treated fish. The gills, liver, kidney and spleen tissues were irritated and injured causing dysfunction in these organs. Haemoglobin contents were increased in fish toxicated by copper and decreased in those toxicated by lead . Serum glucose was increased in all groups treated with copper or lead. The serm AST and ALT record­ed the same trend of increase . Moreover, the creatinine and the uric acid levels were elevated .