Abstract Theelastic compressibility of gas vesicles isolated from Anabaena flos-aquae has been measured with a specially constructed apparatus. The gas vesicle suspension was contained in a glass tube, closed at one end with a piston allowing volume adjustment and attached at the other end to a microcapillary, and was subjected to pressure from compressed air. The elastic compressibility of the gas vesicle suspension was determined by applying or removing pressure and measuring the ensuing displacement of the meniscus in the capillary with a vernier microscope. After allowing for the compressibility of the compression tube and of water in the suspension, the compressibility of the intact gas vesicles has been calculated to be 0.00155 bar-1, and the elastic bulk modulus 645 bar. The elastic modulus of the protein that forms the gas vesicle wall can also be calculated from these measurements; it is 27500 bar. These measurements confirm that the gas vesicle is a rigid structure and show that the buoyancy provided by them will be relatively unaffected by pressures that do not actually cause gas vesicle collapse. The apparatus described can also be used to provide a direct measure­ment of the volume of gas vesicle gas space present in a suspension of a gas-vacuolate organism, and to investigate the gas vesicle critical collapse pressure. Gas vesicles appear to collapse by instability failure but the pressure at which this occurs, about 6 bar, is higher than would be predicted from knowledge of the dimensions and elastic modulus of the gas vesicle wall. This supports the idea that the orientation of the ribs, which form the structure, provides ring-stiffening support.