Cell membranes of the cyanobacterium Phormidium lamino.sum were separated into two fractions by centrifugation in a stepwise sucrose gradient. One fraction contained mature thylakoid membranes and the other one contained membranes morphologically similar to primary membranes of pea etiochloroplasts. The latter ones represented an electron-transparent base with numerous electron-dense inclusions. The predominant type of the inclusions was rod-like structures containing presumably ribosomes.