The size and location of chloride cells in gill of Hilsa, Tenualosa ilisha, from marine (Digha) and fresh water (Farakka) environments were studied in this paper. Changes in gill and kidney histopathology in terms of Na + , K+ -ATPase activity were also investigated. Fish samples were collected from fresh water (FW) (salinity ~ 0.1psu) and marine water (MW) (salinity ~ 29 psu) environment. In FW, the chloride cells were observed on the epithelium of the filaments (mainly in inter-lamellar regions) and on the lamellae of Hilsa gill. On the contrary, in the MW samples, the abundance of Na+ ,K+ -ATPase increased and few chloride cells were observed on the lamellae. MW samples showed a high density of chloride cells on the epithelium of the filaments, and a few cells on the lamellae. Na+ , K+ -ATPase intensity was significantly differed in two unlike environment (sustainability higher in MW samples compared to FW samples) where kidney samples showed the opposite trend. The capability of T.ilisha to change the number and size of gill chloride cells, as well as their activities indicated that the high degree of adaptability of T.ilisha to a wide range of salinity.
