Abstract: The effects of temperature,light and pH on photosynthesis of Microcystis aeruginosa and Chloromonas rosae were studied by measuring their net photosynthetic O₂ evolutions.The net photosynthesis of both algae increase with the temperature.The net photosynthetic O₂ evolution of M.aeruginosa increased linearly in the range of 10-35℃,indicating that its optimal temperature might be higher than 35℃,while the net photosynthetic O₂ evolution of Ch.rosae decreased dramatically when the temperature was over 30℃.The photosynthesis-light response curves of these two algae are different.The light saturation point(LSP) of M.aeruginosa was around 500 μmol·m^-2·s^-1 and photoinhibition did not occur when the light intensity was as high as 900 μmol·m^-2·s^-1,much different from M.aeruginosa,photosynthesis of Ch.rosae was inhibited when the light intensity exceeded its LSP of 630 μmol·m^-2·s^-1.The photosynthesis of M.aeruginosa,with an optimal pH of 10.0,was very active and stable in the range of pH 6.5-11.5.The optimal pH of Ch.rosae was 7.0,and any slight departure from the optimal pH would significantly reduce its photosynthesis.When the value of pH exceeded 10.0,its net photosynthe-tic O₂ evolution was negative.M.aeruginosa has three important characteristics that are different from Ch.rosae:(1) a wider tolerable temperature range,adaptation to high temperature;(2) lower LSP,higher photosynthesis activity,and more efficient photosynthesis under low light environment;(3) significant pH tolerance,and high photosynthesis activity in neutral and alkaline environment.All these biological characteristics give M.aeruginosa great competition advantages to survive,grow and dominate other phytoplankton in polluted aquatic environment.