The membranous systems of the "chloride cell" were studied in teleostean fish gills stained in ferrocyanide-reduced osmium (Karnovsky, '71). Three distinct systems were observed: (1) the tubular system, densely stained with ferrocyanide-reduced osmium, was made up of anastomosed tubules opening in the latero basal intercellular space; (2) the endoplasmic reticulum, faintly stained, and continuous with the nuclear envelope; (3) the vesiculotubular system, the staining of which was intermediate between those of both previously cited systems, was made up of vesicles and short tubules. These membranous systems underwent modifications according to the salinity of the exterior medium: (1) the tubular system formed a broad and loose network in fresh water adapted fishes; in salt water, the meshes of the network became small, tight, and regular thus increasing the cell surface area. (2) the endoplasmic reticulum, which in fresh water, consisted of dilated cisternae often studded with ribosomes; in salt water, it developed in a network of anastomosed smooth sheets interdigitated with the tubular system. (3) the vesiculotubular system seems to be also more developed in salt water than in fresh water-adapted fishes.