Microcystis strains were isolated from water-blooms that had been collected from Korean lakes and reservoirs and cultured under uni-algal, non-axenic condition. Late exponential phase cells were harvested, concentrated and lyophilized for toxin extraction. Isolation, purification and qualitative and quantitative characterization of toxins were carried out by High Performance Liquid Chromatography(HPLC) system. Among 16 tested Microcystis strains, 14 strains produced hepatotoxic heptapeptide microcystins. The amounts and types of toxins were different according to strains. The identified toxins were microcystin-RR, -LR and YR.