Cellular differentiation and renewal in the gill chloride cells were examined in freshwater (FW) and seawater (SW)-adapted chum salmon (Oncorhynchus keta) fry and in fry during SW adaptation using 5-bromo-2′-deoxyuridine (BrdU) as a marker for newly-differentiated cells. Chloride cells and BrdU-labeled nuclei were immunocytochemically detected by using antisera specific for Na ,K -ATPase and BrdU, respectively. Although the number of chloride cells located at the base of the lamellae and in the interlamellar region (filament chloride cells) was constant in FW, SW and SW-transferred groups, chloride cells located in the lamellar epithelium (lamellar chloride cells) were fewer in SW than in FW, and decreased during SW adaptation. Newly-differentiated cells with BrdU-immunoreactive nuclei were detected mainly in the filaments, and rarely observed in the lamellae. The turnover rates of filament chloride cells for FW, SW and SW-transferred fish during the first 24 hr were 8%, 21% and 28%, respectively. These results indicate that chloride cells in the filament are replaced continuously by newly-differentiated cells in both FW and SW, and that the turnover was about 3 times greater in SW than in FW. More frequent turnover of filament chloride cells in SW suggests a specific role, presumably in salt excretion.