To clarify an osmoregulatory role of branchial chloride cells in successful seawater adaptation of newborn guppies Poecilia reticulata, the present study examined the functional and morphological changes in branchial chloride cells during freshwater and seawater adaptation using immunohistochemical localization of Na + ,K + -ATPase. In the newborn guppies of the freshwater-adapted strain, chloride cells were located on the primary lamellae and showed strong immunoreactivity for Na + ,K + -ATPase. During adaptation of the newborn guppies to freshwater, their salinity tolerance decreased within five days after birth. Although the total number of chloride cells did not change, the immunoreactive chloride cells decreased in number and size and the immunonegative chloride cells appeared on the primary lamellae. In the seawater-adapted strain, on the other hand, the newborn guppies had larger chloride cells and higher salinity tolerance than those of the freshwater-adapted strain. Acclimation of the mothers to freshwater caused reductions in the chloride cell size and the salinity tolerance of their newborn guppies, indicating that the mother's environmental salinity enhanced the chloride cell size and the salinity tolerance of the newborn guppies in the seawater-adapted strain. During adaptation of the newborn guppies to seawater, their salinity tolerance increased within five days after birth. All of the chloride cells showed a strong immunoreactivity and the cell size increased with no changes in cell number. During the freshwater and seawater adaptation, the salinity tolerance significantly correlated with the sectional area of the immunoreactive chloride cells in the gills. These results indicate that the osmoregulatory activity of the chloride cells, expressed as the immunoreactivity for Na + ,K + -ATPase and the cell number and size, is important for the successful seawater adaptation of newborn guppies.