1. Under laboratory conditions, the mantle cavities of Lymnaea stagnalis and Planorbarius corneus immediately after ventilation at the surface, contain enough air to make the animals buoyant. 2. Both species lose this buoyancy over the first three hours of submersion by a process reducing the gas volume. 3. The gas is used as an air store and as a physical gill. 4. There is no evidence to suggest buoyancy adjustment, either by gas secretion or by compression of the bubble during periods of submersion. 5. The buoyancy of the shell and visceral mass determines the amount of gas uptake at the surface, and may provide the stimulus releasing the surfacing behaviour of the freshwater pulmonates.