The colonial gas-vacuolate cyanobacterium Anabaena lemmermannii has formed modest populations in Windermere during the months of June and July in recent years. During the summer of 1990 the colonies were dispersed in the well-mixed upper epilimnion of the main lake basins but formed light surface scums along leeward shores and in sheltered bays, including Mitchell Wyke. The gas vesicles of this species were found to be much stronger (p(c)0.93 MPa) than those in other Anabaena species; they would withstand exposure to hydrostatic pressures at depths exceeding 40 m and they would therefore survive deep-mixing events in this lake. The colonies were highly buoyant; they required only 46% of their gas vesicles to render 50% of the colonies neutrally buoyant in one sample. In another sample the buoyant density of colonies with intact gas vesicles was 975 kg m-3 compared with 1022 kg m-3 after gas vesicles collapse, which indicated that only 51% of the gas vesicles were need to give the same density as water. In a third sample comparison of the mean colony floation rate of 181-mu-m s-1 before gas vesicle collapse with the sinking rate of 186-mu-m s-1 after collapse also demonstrated that 51% of the gas vesicles were required for neutral buoyancy. Colonies incubated in two bottles at the lake surface failed to respond to the high irradiance by losing their buoyancy. It was calculated that the colonies would not have been able to accumulate sufficient carbohydrate in a day to overcome the excess buoyancy provided by their gas vesicles; the high critical pressure of the gas vesicles prevented their collapse by turgor pressure rise. The inability of highly buoyant cyanobacteria to lose buoyancy may provide a general explanation for the persistence of cyanobacterial surface water-blooms.