Most cell junctions of Sycon are of the simple kind in which the plasma membranes are separated by an intercellular space of about 15 mmu. True septate junctions are found between sclerocytes. They are similar to those of coelenterates and echinoderms, with a parallel arrangement of septa as seen in tangential sections. The functional implications of the restriction of this type of junction to the sclerocytes are discussed.