In teleosts the head region, particularly the gills, plays the key role in osmoregulatory NaCl transport, presumably by mechanisms located in the chloride cell. As interest has focused on specific mechanisms of chloride cell function, two classical preparations, the intact fish and the isolated, perfused gill, have continued to serve as the only available model systems. However, both of these preparations have severe limitations, e.g., as they are not flat sheets, they cannot be studied readily under the ideal thermodynamic conditions achieved with the short-circuit current technique. The present sutdy describes the histology and ultrastructure of a particular area of skin in the head region of both killifish (Fundulus heteroclitus) and sea raven (Hemitripterus americanus). This skin lies on the inside of the operculum and possesses a flat epithelium containing chloride cells. In the present study the identity of the chloride cell in this epithelium was definitively established with the electron microscope. Although the opercular epithelium from the marine sea raven contains few chloride cells, that from the euryhaline killifish adapted to pond water, 100%-, and 200% artificial seawater is predominately chloride cells. Significantly, the teleost gill has never been reported to contain more than 10% chloride cells. Thus the opercular skin of the killifish can serve as a model to study the adaptive role of chloride cells in euryhaline teleosts. A separate electrophysiological study has deminstrated that the short-circuit current technique can be applied to this skin.