At the Saint-Caprais reservoir (France), a mono-specific bloom of Aphanizomenon flos-aquae occurs every year in the autumn months. Levels of microcystin-LR (MCYST-LR) in this reservoir were evaluated by protein phosphatase type 2A (PP2A) inhibition test as MCYST-LR equivalents in both raw and drinking water. Analysis by HPLC of the crude extract of the mono-specific bloom of A. flos-aquae revealed the presence of MCYST-LR with a low concentration of 270.3 +/- 20.4 ng/g wet weight. MCYST-LR equivalent concentrations in raw water were correlated with the cyanobacteria biomass and they varied between 14 and 74 ng/l. The removal of A. flos-aquae and microcystins were evaluated in a small full-scale plant associated with the Saint-Caprais reservoir. Total elimination of cyanobacterial cells and the low concentration of hepatotoxins was achieved through the combined action of pre-ozonation at 0.07 mg/l and adsorption on powdered activated carbon at 20 mg/l. However, pre-chlorination at 0.42 mg/l followed by 20 mg/l of powdered activated carbon removed only 45% of hepatotoxins.