The short-circuited opercular epithelium of seawater-adapted killifish Fundulus heteroclitus was studied following addition of zinc to both serosal and mucosal sides of the preparation. Zn inhibited the short-circuit current which is a direct measure of the chloride transport across the epithelium. Light microscope observations showed that the mucosal side of the epithelium was completely desquamated after 1 h exposure to Zn. It is suggested that desquamation of gill epithelia described by several authors following in vivo Zn contamination is a direct consequence of the breakdown of osmoregulatory processes. © 1984.