Gills and kidneys alterations of Oreochromis aureus (L.) fingerlings induced by variations of salinity tolerance (5, 10 and 20 ‰) for about 105 days were documented in this study. The fish reared at low salinity concentrations showed gills with epithelial cellular hyperplasia and no changes in the kidneys. Meanwhile, fish held in high salinity concentrations showed congestion of the basal lacunae of the respiratory lamellae. Telangiectatic lamellae were also observed. In addition, the distal convoluted tubules of the kidney showed marked degeneration and appeared as if they were ghosts of tubules. Generally, glomerular swelling, congestion, degeneration and reduced volume of hemopoietic tissues were associated with increasing the salt concentration. The results also revealed that, body weight gain and the specific growth rate were increased until 60 days and decreased by 75 days. In conclusion, the growth rate of O. aureus (L.) increased by increasing the salinity degree up to 20 ‰ and this species can successfully be acclimatized to sea water with gradual continuous acclimatization period of 60 days. In addition, the salinity- induced alterations in gills and kidneys seem to reflect physiological adaptation to the new environment. ____________________________________________________________________