Abstract: Bloom- forming cyanobacteria Microcystis aeruginosa and M.viridis were isolated and purified from the eutrophic lake of Dianchi, Kunming in Southwestern China.At temperature 25℃ and light intensity at 80μE, the growth rate of M.aeruginosa and M.viridis were 0.61 and 0.63, respectively.It was shown that light intensity over 100μmol m-2s-1 had inhibited the growth of both species.In M.aeruginosa, three microcystins, MYCST-RR, MYCTS-YR and MYCST-LR were identified as major toxins; M.viridis produced two major toxins- Dha7-MYCST-RR and Dha7-MYCST-LR, with Dha7 -MYCST-YR as the minor component.It was revealed that light played an important role in regulating the contents of toxins, the highest toxin contents was found in low light intensity at 15μmol m-2s-1, while the lowest ones occurred at higher light intensity at 100μmol m-2s-1.The variation of temperature had little effect on toxin production.The implication of the chlorophyll fluorescence ratio of Fv to Fm in reflecting the cell ability to adapt light intensity has also been explored .