The morphological parameters of the cylindrical gas vacuoles in the blue-green alga, Microcystis aeruginosa, were investigated by quantitative electron microscopy. The gas vacuoles are made up of a single layer of particles of 2.8 × 4.2 × 3 nm size. The particles are linearly arrayed in ribs that wind helically around the main axis of the organelle. The diameter of the cylinders is 71.1 ± 2.7 nm and their length is in the range of 360 ± 90 nm. The gas vacuoles can be flattened out under hydrostatic pressure.