The gas vacuoles which occur in various prokaryotic organisms can be estimated quantitatively by the change in light scattering which takes place when they are destroyed by pressure. The gradual disappearance of gas vacuoles under rising pressure is explained by the intrinsic variation in critical collapse pressure of their constituent gas vesicles. These collapse instantaneously at pressures exceeding their critical pressure, but withstand repeated and prolonged application of pressures below this value. Gas vesicle membranes are freely permeable to gases, and as a consequence the vacuole gas is at atmospheric pressure in aerated suspensions. Increasing or decreasing the pressure of gas in the gas vacuoles brings about a corresponding change in the pressure required to collapse them, indicating that the vacuole gas helps to support the structure. Pressures in excess of the vacuole gas pressure are borne by the membrane itself. The pressure required to collapse gas vacuoles present in blue-green algae is increased if the cells are suspended in a hypertonic sucrose solution, because this removes the cell turgor pressure acting on them. This observation, which confirms the classical theory on the osmotic relationships of plant cells, provides the first reliable method of estimating turgor pressures in prokaryotic organisms. Cell turgor pressure was found to be higher in a blue-green alga than in a purple sulphur bacterium investigated; no cell turgor could be detected in a halobacterium which grows in saturated brines, suggesting that the salt concentration must be the same inside and outside the cell. The gas vesicles in these organisms seemed to be adapted to withstand the pressures they were likely to encounter, those of the alga being the strongest, and those of the halobacterium the weakest. Even so, the range of turgor pressure overlapped the critical pressure range of the gas vesicles in the alga and purple sulphur bacterium so that turgor pressure alone may effect their collapse under certain circumstances. With the alga this seems to happen in conditions promoting photosynthesis, providing the organism with a means of regulating its buoyancy. It is suggested that the width of a gas vesicle is important in determining its strength, and that this explains the differences in size and shape of the gas vesicles which have evolved in the three organisms. Interfacial tension could in theory exert considerable pressures on the highly curved surface of a gas vesicle but this effect would be minimized if its outer surface were of a hydrophilic nature. Several observations have been made which support this idea. Pressures generated by centrifugation will collapse isolated gas vesicles and must be considered when using this technique to purify them. Sufficient pressure to collapse gas vesicles can also be developed in small columns by the massive negative accelerations developed in collisions. This phenomenon, which may have application in engineering fields, must also be reckoned with in handling these pressure-sensitive structures. It is concluded that even though the gas vesicle membrane must tear during its collapse, the gas it contains diffuses away rather than escaping as a bubble.