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Cytokinesis in the unicellular chlorococcalean alga Eremosphaera viridis de Bary has been investigated by electron microscopy of thin sections. The new plasmalemmata of the daughter cells in this organism form centrifugally within a phycoplast. Unlike other cell division systems each new plasmalemma is formed, not by the fusion of vesicles, but rather by the fusion of "open membranes" which are characteristically heavily stained. Measurements of these "open membranes" reveal that they are 11 nm thick with a central 4,5 nm unstained portion. The possible origin of these "open membranes" as burst-open vesicles has been suggested from the presence of intensely straining vesicles in the vicinity of the cell equator. Calculations of vesicle and "open membrane" surface areas support this contention.