Seasonal changes in the biomass of Microcystis species were investigated during 1982-1985 in Lake Kasumigaura, Japan. Microcystis in this lake was composed of four formas and species, i.e., M. aeruginosa f. aeruginosa, M. aeruginosa f. flos-aquae, M. viridis and M. wesenbergii. These taxa almost always appeared as a mixed population. The dominant species did not vary largely, station by station, but changed remarkably year by year. M. aeruginosa f. aeruginosa and M. aeruginosa f. flos-aquae tended to predominate in the early stage of the bloom or throughout the bloom, while M. viridis did so in the middle and late stages of the bloom. M. wesenbergii did not appear with a definite tendency. An over wintering population in the water was composed of M. aeruginosa f. aeruginosa and M. viridis. The importance of studies on Microcystis at the species level was discussed.