Teleosts, the bony fishes, inhabit both freshwater and seawater environments. Some euryhaline fish, such as Fundulus heteroclitus, alternate between the two milieux several times daily. Regardless of adaptation, the gills of these animals possess a highly specialized cell type called the chloride cell. This cell contains numerous mitochondria and exhibits a greatly amplified basolateral cell surface richly endowed with Na,K-ATPase. Recent studies on isolated opercular epithelia containing chloride cells have demonstrated active chloride secretion and passive transepithelial sodium movements, and have established the chloride secretory role of this cell type in seawater-adapted teleosts. Current models suggest that chloride transport occurs via a transcellular route. Seawater chloride cells exist in multicellular units and share simple, shallow tight junctions which are thought to be the route for passive sodium movement. Freshwater chloride cells, whose function remains to be elucidated, are generally described as existing in a unicellular configuration. However, recent observations in Fundulus heteroclitus adapted to salinities as low as 1% sea water reveal that chloride cells persist in multicellular complexes with apical crypts. Strikingly, tight junctions between chloride cells in this freshwater environment are deep