A strain of bacterium capable of biodegrading microcystin (MC) RR and MC-LR was isolated from the sediments of Dianchi Lake. It was demonstrated that the enzymes in cell-free extract of this bacterium were responsible for the degradation of MC-RR and MC-LR, and the dead-end products of MC-RR and MC-LR catalyzed by these enzymes were observed on HPLC chromatograms. Results show that the optimum pH for the activities of these enzymes was in the range from 6.0 to 8.0, however the metal ions of Cu2+, Mn2+ and Zn2+ had no apparent effects on the enzymatic degradation of MC-RR and MC-LR.