1. The general cytoplasmic features of Fischerella muscicola, Stigonema hormoides and cells of Stigonema mamillosum found as a phycobiont in the lichen Ephebe lanata are similar to those found in other cyanophyte cells. 2. In all instances the outer surface of the quadrilayered cell wall bears a series of ridges. The membrane-like outermost layer (L IV) also exhibits a number of evaginations thought to represent sites of extrusion of material from the cytoplasm through the wall into the enveloping fibrous mucilage. 3. Cross walls are formed by inward growth of the two inner layers of the lateral walls, the daughter cells subsequently moving apart through the inward intrusion of the two outer wall layers and a bulk of mucilage. 4. The daughter cells remain connected by a persistent central region of the original septum. This septum is penetrated by pores, although these do not pierce the underlying plasma membranes and no direct connection of the protoplasts is achieved. 5. An apical cell of S. hormoides was found to be capped by remnants of a cross wall, presumably by the disruption of a previously distal cell.