Mesosome-like, unit-membrane structures are clearly defined in the blue-green algae, Spirulina and three strains of Synechococcus, after osmium or potassium permanganate fixation and observation with the electron microscope. The membranous structures are distinct from the photosynthetic membranes and, in the case of Spirulina, are frequently observed in cells and can occur in large volume within the cell.