Ultrastructural findings in Fish Gills. The ultrastructure of the secondary lamella is observed in gills of the following Fishes: Scyliorhinus stellaris L., Salmo irideus Gibb., Anguilla anguilla L., Carassius auratus L. The secondary or respiratory lamella consists of an epithelial layer covering a vascular axis. Contrary to the current opinions, the epithelium appears to be double layered. The cells of the two layers have a different morphological aspect and their nuclei are arranged in two lines; therefore it seems possible to exclude that the double layered appearence is a consequence of a cytoplasmic strips superimposition. Besides the covering cells, there are mucous cells and "chloride cells". The former are round shaped, and their cytoplasm contains a very developed ergastoplasm, and numerous vesicles full of an electron-trasparent, probably mucopolysaccaridic, substance. The "chloride cells" show a large nucleus, numerous mitochondria and a complex tubular tridimensional system of the smooth endoplasmic reticulum. The function of these cells is probably to secrete sodium and potassium chloride.