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Abstract. The objectives of this study were to evaluate the effects of sub-lethal lead concentrations on serum osmolality, Na + and Cl -levels, and hematological parameters in Nile tilapia, Oreochromis niloticus (L.) at different salinity levels. The serum osmolalities (SO) were not significantly different at any of the salinity levels in the control fish, while in Pb-exposed fish the SO increased with increasing salinity. The concentrations of serum Na + and Cl -in both the control and Pb-exposed fish increased with increasing salinity. The levels of red blood cells (RBC), hemoglobin (Hb), and hematocrit (Ht) in the control fish were not significantly different at any of the salinity levels. Meanwhile, the levels of RBC, Hb, and Ht in Pb-exposed fish increased with increasing salinity levels. The levels of RBC (at 0 and 5 ppt) and Ht (at 0, 5 and 10 ppt) in Pb-exposed fish were lower than in the control fish. The levels of Hb in Pb-exposed fish were lower than in the control fish at all salinity levels. The levels of WBC in the control fish increased with increasing salinity, while its levels in the Pb-exposed fish decreased with increasing salinity. The levels of WBC in the Pb-exposed fish were higher than in the control fish at 0 and 5 ppt.