Abstract: The present study elucidates, the relationship between chloride cells and salinity adaptation in the guppy, Lebistes reticulatus. Guppies were exposed to different salinities (2, 5, 8 and 11%) for 2 weeks to adapt them to salt water with the density of chloride cells was examined by light microscopy. The chloride cells of the gills were selectively stained by the fixator of Champy’s modified by Maillet. The chloride cell density increased significantly in fish adapted to 8 and 11% salt concentration, which suggests that the densities of branchial chloride cells increased with environmental salinity.