Abstract: This study shows gradual acclimation of 40 fresh water red hybrid tilapia O. mosssambicus X O. hornorum (salinity = 0.1%) to marine water (42%) during a period of 2 weeks. The adapted fishes (80%) tolerated living in marine water for 7 days. The histological sections of gills, liver and kidneys clears the appearance of chloride cells only in the gills of the marine water adapted fishes. Sections of the liver contain bile granules within the hepatic cells of the liver of the adapted fishes and the histological sections of their kidneys shows dilatation in the tubular lumen