In the summer of 1999, a bloom (11 100 filaments ml-1) of the gas vacuolate cyanobacterium Aphanizomenon ovalisporum developed in a shallow (1.7 m deep) reservoir containing nutrient-enriched water from Lake Kinneret (Israel). During 4 days, A. ovalisporum showed a marked diel periodicity in buoyancy: the proportion of floating filaments fluctuated between 76-84% from midday to evening and 94-98% at the end of the night, in both surface and bottom samples. Buoyant filaments were present throughout the water column, presumably due to wind-driven vertical mixing. Aphanizomenon filaments collected from the reservoir were maintained under mean photon irradiances of 15 (LL), 150 (ML) and 11 00 (HL) μmol m-2 s-1 in a computer-controlled set-up, which simulated the diel light changes at different depths in the reservoir. In the LL cultures, filament buoyancy showed no diel fluctuation patterns during the 4 days of incubation, but ML and HL cultures showed regular diel changes, with a higher proportion of filaments floating at the end of the night than during midday-evening. There was no evidence for either turgor-driven collapse of gas vesicles or dilution of gas vesicles by cell growth by any of the treatments. Gas vesicles of A. ovalisporum had a relatively low mean critical pressure (pc of 0.57 MPa), but the daytime rise in turgor pressure was too small to cause gas vesicle collapse. The observed diel buoyancy changes may be explained by accumulation of carbohydrate ballast during the day and decrease during the night.