Abstract: Effects of organic phosphor pesticide DDV on blooming of cyanobacterium Microcystis were studied.Growths of Microcystis aeruginosa PCC7806,a unicellular strain and Microcystis aeruginosa XW01,a colonial strain,were investigated as affected by different concentrations of DDV.Results show that the growth of Microcystis was inhibited by DDV high in concentration,but promoted by DDV low in concentration in BG11 culture medium.PCC 7806 was more sensitive to DDV than XW01.The effect was more obvious in the culture medium deficient in phosphate than in the BG11.The increased growth did not result from phosphor supplement by DDV,but DDV stimulated activities of the intracellular and extracellular alkaline phosphatase of Microcystis,which suggests that once DDV residue enters a water body,it may promote formation of Microcystis bloom.