Ultrastructural alterations of the chloride cell of the dogfish (Scyliorhinus canicula) gill epithelium were studied following a short-term exposure of the fish to lethal doses of zinc. After treatment, mitochondria adopted cup-shaped and ring-shaped profiles with the cristae orientated parallel to the long axis of the organelle. Cytolysosomes increased as well in number as in size which might be related to the digestion of altered mitochondria. We suggest that these alterations are probably related to an impairment of oxidative phosphorylation which leads to a blockade of the Na, K-pump. © 1986.