Fish productivity has been affected by a progressive increase of salinity and pollution in Lake Qarun. So. this work aimed to investigate the histology and ultrastructure of the gills of Tilapia nilotica to clarify the possible effect of water quality on the fish gills. The most observed changes include the occurrence of a prominent large number of chloride and mucous cells confined to gill filament, hyperplasia of primary lamellar epithelium which leads to obliteration of some interJamellar spaces between adjacent secondary gill lamellae, odema resulting in the separation of respiratory epithelium and inflammatory infiltration including eosinophils and macrophages. These findings demonstrate that Tilapia nilotica can adapt to poor conditions through cellular and histological changes.