To elucidate the ultrastructural modifications of the gill epithelium during smoltification, gills of the Atlantic salmon (Salmon salar ) were examined by electron microscopy at three stages of this process: parrs, freshwater smolts, and seawater smolts. In the gill epithelium of parrs, there were two types of chloride cells. In freshwater smolts, the large chloride cells formed 95% of the chloride-cell population. It was concluded that the Atlantic salmon develops in fresh water most of the ultrastructural modifications of the gill epithelium which in most euryhaline fish are triggered by exposure to seawater. The effective transfer into seawater would act only as a final stimulus to achieve some adequacy between the freshwater smolt and its new environment.