Junctions between the cells in the reticular lamina of the organ of Corti were examined in thin sections and after freeze-fracturing to find a structural basis for the large ionic differences between the endolymph and perilymph. The apices of the cells in the reticular lamina are joined by a band of tight junctions spaced at 140 A intervals. Beneath this apical band the organization of the tight junctions depends on whether they join a supporting cell and a hair cell, or two supporting cells. At hair cell junctions with supporting cells, there is an extensive labyrinth of tight junctions enclosing lengthy, tortuous passages whose walls are composed of either multiple parallel or single junctions. At appositions between two supportinc cells, maculae or fasciae occludentes lie immediately beneath the apical bands of closely spaced tight junctions, near the top of the zonulae adherentia which are characteristic of appositions between supporting cells. The complexes of tight junctions, or zonulae occludentes, between extralaminar supporting cells differ from those in the reticular lamina. The extralaminar cells are joined by a band of four to seven branching, anastomotic tight junctions. Thus, these junctions are like zonulae occludentes in other tissues. The novel organizations of the tight junctions in the reticular lamina, different from those between the extralaminar supporting cells, suggests a special role for these junctions in the reticular lamina. Two sizes of gap junctions link, and presumably couple, supporting cells in the reticular lamina.