Summary. Degeneration and death of branchial epithelial cells were studied in an African cichlid fish. In both fresh water and seawater fish the superficially located pavement cells are sloughed off at the end of their lifecycle. This process is preceded by degeneration via a process of cyto plasmic shrinkage and condensation related to apoptotic (physiologically controlled) cell death. The chloride cells are pleomorphic, i.e., accessory, mature, and degenerating cells. Degeneration of chloride cells mainly occurs by apop tosis. Degenerating cells show shrinkage and densification of cytoplasm and nuclei, and swelling of the tubular system; these cells are then separated from the ambient water by pavement cells. They are finally phagocytosed and digested by macrophages. Apoptosis of chloride cells, but not of pavement cells, is greatly stimulated when the fish are in seawater; this reflects an increase in cellular turnover of the chloride cells. Accidental cell death (necrosis) of pave ment cells or chloride cells is rarely observed in fully adapted freshwater and seawater fish. Its incidence in creases in the first few days following transfer of fish from fresh water to seawater.