The present study investigated the cyanobacteria and one family of their toxins-microcystins (MCs) in Guanting Reservoir of Beijing, China. The dominant species in the cyanobacteria found in August and September of 2006 was Microcystis, which accounted 99% of total algal cells. The specific species of the Microcystis in the cyanobacteria included Microcystis ichthyobalbe, Microcystis novacekii, Microcystis botrys and Microcystis aeruginosa which had different ratios in different sites. The qualitative analysis by HPLC showed that two microcystins were contained in cyanobacteria and one microcystin was in water of the reservoir. The major microcystins were microcystin-RR (MC-RR) and microcystin-LR (MC-LR), but only MC-LR was detected in water. The quantitative analysis by HPLC indicated that the maximum concentrations of MC-RR and MC-LR contained in cyanobacteria were 0.74 and 0.41 mg/g dry weight, respectively. The maximum microcystin concentration in water was 1.15 mu g/L and others were below 1 mu g/L. (c) 2007 Published by Elsevier B.V.