Abstract The average thickness of the layer of protein which forms the wall of the gas vesicles in Anabaena flos-aquae was estimated from measurements of their density and geometry. The volume of the gas space in a purified gas vesicle suspension was determined from the contraction which occurred when the gas vesicles were collapsed by pressure. The volume of the protein in the same sample was calculated from its dry weight and density. From knowledge of the geometry of the average gas vesicle the thickness of the protein layer, 1.54 nm, was then calculated. By a similar method the thickness of the Microcystis gas vesicle wall, 1.62 nm, was calculated from data published by others. The average thickness of the protein layer is, as expected, slightly less than the stacking periodicity of collapsed gas vesicle walls indicated by X-ray diffraction studies. Anabaena gas vesicles with a mean length of 494 nm have an average density of 0.119 mg μl −1 1 mg of protein is present in gas vesicles having a, total volume of 8.43 μl and a gas space of 7.67 μl. Suspensions of isolated gas vesicles with a gas space concentration of 1 μl ml −1 give a pressure-sensitive optical density, E 1cm (500 nm) of 2.72, but gas vacuoles in cells give a smaller value.