Colonies of Microcystis aeruginosa have dominated the phytoplankton in Lund tube C, a limnetic enclosure in Blelham Tarn, English Lake District, during the summer and autumn in recent years. Following holomixis in autumn the previously buoyant colonies sedimented from the water column onto the bottom mud. In all samples gas vesicles, which provided the colonies with buoyancy, were present in sufficient volume to negate the combined ballast provided by protein, carbohydrate, lipid and phosphate, the major cell components. The gas vesicles, which accounted for about 10% of the cell protein, were too strong to be collapsed or regulated by cell turgor pressure. Consequently, the loss of buoyancy could not be explained by an increase in cell ballast or by disappearance of gas vesicles. Colonies collected in sediment traps were found to be buoyant after they had been agitated and diluted with lake water, which removed a colloidal precipitate from them. Similarly, 66% of the sinking fraction of a net tow sample was found to be buoyant after it had been treated in the same way. Previously buoyant colonies could be made to sink on mixing with the colloidal precipitate. This demonstrated the ability of the precipitate to trap colonies and to cause their sedimentation. The colloid comprised approximately equal amounts of organic and inorganic matter and was rich in iron. Colloids of this type form when the dissolved iron in the anoxic water of the hypolimnion becomes oxidized on mixing with the aerated water of the epilimnion.