Freeze-fracture replicas of sponge cell membranes revealed in general a low density of intramembranous particles, with the exceptions of the membrane (silicalemma) surrounding the siliceous spicules in Ephydatia and the membranes of spherulous cells in Chondrosia. In addition, several types of particle arrangements were observed. A classical necklace is present at the base of the choanocyte flagellum. Rosettes of particles are particularly obvious in the apical membranes of choanocytes, where they are associated with the fuzzy coat covering these cells. Parallel ridges of particles were observed along the microvilli of the choanocyte collar, at sites of insertion of connecting filaments. Rows of particles were observed in the plasma membrane of pinacocytes in Ephydatia where they are located on areas deformed by protruding fibrillar inclusions. Pinacocyte plasma membranes in this species also can contain accumulations of particles which are likely related to desmosomes. Single rows of aligned particles and double rows of staggered particles (sometimes organized in large plates) in addition to rhombic particle arrays were encountered on replicas of marine sponge cell membranes. No classical arrangements corresponding to gap junctions, tight junctions or septate desmosomes were observed. The significance of these data is analysed.