Since two years, the lake of Bourget (France) which is used for the water supply to several agglomerations, has known problems with cyanobacterial blooms. A first bloom implying Planktothrix rubescens, was observed from November 1998 to March 1999. In less than two weeks, the cell number increased to 20.000 cells ml-1 and the filament dry mass to more than 1.4 mg l-1. The vertical distribution of the filaments was homogeneous above the thermocline, through the depths of 0-45 m, and the horizontal distribution was also homogeneous on the lake. At this time, more than 80 % of the biomass was constituted by P. rubescens. A first decrease of the cell number per milliliter was observed at the end of December. This decrease fits to a drop of the thermocline. In January, the temperature of the water mass of the lake was homogeneous (from 0 to 140 m) and the density of cynobacterial was also homogeneous from the bottom to the surface (10.000 to 15.000 cells ml-1). At the middle of February, a second decrease was observed until March, at the end of the bloom. In July 1999, a strong increase of the cyanobacterial biomass was observed with 10 meters of depth. Two species were present at the beginning of the bloom: P. rubescens and Aphanizomenon flos-aquae. From July to the middle of September, the cyanobacteria were principally located in the metalimnion, between 10 to 15 meters from depth. After that, the vertical distribution of cyanobacteria became more homogeneous above the thermocline and Pseudanabaena sp. and Oscillatoria limnetica substituted A. flos-aquae. Toxicity of these blooms was analyzed by HPLC. Two forms of the microcystin-RR were mainly observed. The amount of microcystin per milliliter was correlated to the number of cells per milliliter and more than 4 µg/l of microcystin was observed after cell extraction, in water at different times during the blooms. The role on the dynamic of the blooms, of several factors like the interspecific competitions and the potential incidence of these blooms on the human health would be discussed.