A survey of the distribution of cyanobacterial blooms in the southern part of Belgium, in Luxembourg as well as in bordering northeastern France was carried out for 4 years (1997, 1999-2001). In the 64 cyanobacterial bloom samples collected, Microcystis as well as Planktothrix were the most frequently encountered dominant bloom formers, followed by Anabaena, Woronichinia, and Aphanizomenon. The relative frequency of (co-)dominant genera was highly correlated to the geology of the catchments. Microcystins were found in 53% of the analysed blooms and their presence was mainly assigned to Microcystis dominance. The highest microcystin concentration of 2231 μg g-1 seston DW was recorded in a sample dominated by Woronichinia naegeliana. Among the 6 investigated microcystin variants, MC-LR was the most frequently detected whereas MC-LY was never revealed.