ABSTRACT The histology of the gills of a normally freshwater teleost, the guppy (Lebistes reticulatus), has been studied after the fish had been subjected to various concentrations of sodium chloride. Cells similar to those identified by previous workers as ‘chloride- secretory cells’ develop in response to adaptation to hypertonic salt solutions, and the number and character of the cells can be related to the salt load and to the degree of adaptation of the fish to it. The cytology of these cells has been shown to be consistent with their supposed function. In fish which are not fully adapted to the salt load, these cells show signs of the formation of mucous materials, and under extreme conditions they may undergo transformation into goblet cells. This may provide an explanation of the failure of Bevelander (1935, 1936) to find any specialized cells other than mucous cells on the teleost gill.