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The permeability of intact gas vesicles, isolated from the cyanobacterium Anabaena flos-aquae, to percyclofluorobutane (C4F8), has been investigated by two methods. Warburg manometry showed that a suspension of intact gas vesicles exchanges more C4F8 than an equivalent suspension of collapsed ones. Pressure nephelometry showed that increasing the partial pressure of C4F8 in solution caused a corresponding increase in apparent critical collapse pressure of the gas vesicles. From these observations it is concluded that C4F8 is able to permeate freely through the gas vesicle wall. If it permeates via pores, these must be at least as large as the gas molecules, which have a collision diameter of 0·63 nm. Warburg manometry showed that the volume of C4F8 exchanged exceeds the volume of gas-vesicle gas space, and this indicates that, in addition, there is specific absorption of the gas, probably on the inner, gas-facing surface of the gas vesicle wall.