Physiological evidence was obtained for a light shielding role for gas vacuoles in Microcystis aeruginosa Kuetz. emend. Elenkin, by comparing photosynthetic oxygen evolution, growth behaviour and pigment composition of cells with intact or collapsed gas vacuoles. The oxygen evolution rates were strongly dependent on cell concentration, a maximum rate for cells with intact gas vacuoles occurring at about 1.4 X 10(9) cells/ml and for cells with collapsed gas vacuoles at about 2.5 X 10(9) cells/ml. By using light saturation curves for oxygen evolution, it was estimated that at low light intensities up to 30% of the photosynthetically useable light was shielded at a cell concentration of 6 X 10(8) cells/ml. Collapsing the gas vacuoles twice daily did not alter the initial growth rate of the cultures, but enabled them to reach a higher final cell density. Collapsing of gas vacuoles during growth for about four generations resulted in a lower level of all acetone soluble pigments with a greater relative reduction in carotenoids than in chlorophyll a. Collapse of the gas vacuoles does not alter the cell volume. Various optical interactions which could account for light shielding are discussed.