The carbonic anhydrase (CA) activities were determined in three cyanobacterial species, namely Microcystis aeruginosa Kütz, Microcystis viridis（A.Br.）Lemm, Microcystis wesenbergii (Kom.) Kom, which are dominant in a lake (Lake Dianchi) subject to major blooms. In more detailed experiments on M. aeruginosa, the effects of inorganic carbon, pH, temperature, nitrogen/phosphorus ratio, glucose, and light intensity on CA activity were also investigated. The results showed that the CA activity of M.aeruginosa was influenced dramatically by the concentration of bicarbonate. Consequently, it is suggested that bicarbonate ions were the main form of inorganic carbon M.aeruginosa could utilize. Cultures grown in the dark exhibited CA activity 6 times higher than that of cells cultured mixotrophically with the addition of glucose. Features of eutrophic water bodies promote an increase in CA activity, and the resulting higher CA activity would accelerate the utilization of inorganic carbon and favour the growth and blooming of Microcystis spp.in eutrophic lakes. Although the experiments were carried out under controlled experimental conditions, they may provide some basic data that will prove useful for the control of cyanobacterial blooms in nature.