The effects of chronic exposure to sublethal doses of mercuric chloride (0.310, 0.155 and 0.078 mg L"1) on growth rate, physiological parameters, chemical composition of tilapia species and properties of ponds water were studied for 180 days. Tilapia species (Oreockromis niloticus, Saroiherodon galilaeus and Oreochromis aureits) with an average initial size of 13 cm for total length and 50 g for body weight were stocked in fiberglass tanks (6 m2 area of each) at a rate of 15 fish m*2 (1:1:1) and fed on artificial diet containing 30 % protein. The physico-chemical properties of ponds water were slightly varied with different mercury levels. The plankton organisms (phyto - and zooplankton) were gradually decreased with increasing the mercury concentration in fish ponds. The final body weight, daily weight gain and production of tilapia fish were significantly decreased with increasing the mercury level. The lowest values of these parameters (17L5g, 0.67g, 1.518 ton feddan"1 for O. niloticus; 144g, 0.52g, L053 ton feddan"1 for 5. galilaeus and 153.5 g, 0.56 g and 0.995 ton feddan * for O. aureus) were observed in the pond containing highest mercury level (0.310 mg/L). The other growth performance (specific growth rate, percentage weight gain and normalized biomass index), feed utilization (feed conversion ratio, protein efficiency ratio and protein productive value) and survival efficiency of tilapia were also affected with sublethal levels of mercury. The net returns and profitability of the reared fish were also decreased with increasing mercury concentration in ponds water. The lowest hematocrit (28.5, 29.0 and 30 %) and hemoglobin (5.5, 5.0 and 52 g/100 ml blood) of the three fish species were obtained from the pond containing 0.310 mg/L HgCh. At the same mercury levels, the serum protein of the three fishes decreased to 7.0, 6.6 and 6-5 g/100 ml serum, while serum glucose increased to 8L0, 79-0 and 78.5 mg/100 ml serum. The biochemical composition of fish muscles was also influenced with different sublethal levels of mercury, where the muscle protein and glycogen were slightly decreased and lipid increased with increasing the mercury level. The mercury residues in liver, gills and muscles of the reared fish were increased with increasing the mercury concentration in ponds water and its value in liver was higher than that in gills and muscles.