Allelopathy, the release of extracellular compounds that have an effect on the growth of other microorganisms, may be one factor contributing to formation and/or maintenance of cyanobacterial blooms. In this paper, the allelopathic effect between single-cell Microcystis aeruginosct and three filamentous cyanobacteria助hanizomerznn flos-uquae, Anabaena flos-aqtuae, Phormidium mucicolu was investigated. The experiment was conducted to find out the allelopathic effects of cell-free filtrate on the growth of different target organisms; 1.the cell-free filtrate of M.uPruginnsa on Aphunizomenorz flos-aquue, Arzubaerz.a fins-aquae and P. mucicola;2. the cell-free filtrate of Aphurztzomenon flos-uquae, Arzabuena flos-aquae and P. mucicola on M. aeruginosu. The results demonstrated that M.ueruginosu could accelerate the growth of Aphanizontenorz flo.s-rzquae and Anahrzerza flos-uquae, and slightly inhibited P. ntucicola. On the other hand, the growth of M, deruginostx was slightly suppressed by Aphurzizumenorz fins-aquue and.4rzabaena flo.s-aquae, but heavily inhibited by P. mucicolu. The inhibition by P. ntucicolu was enhanced with thf; increase of concentration of cell-free filtrate. Furthermore, the allelopathic effect between single-cell M. uerugino.su and three filamentons cyanobacteria.9phurzizurrzertnn flos-uquoe, Atwbaena floc-aquue, P. mucicolu was not influenced by the change of the concentration of inorganic nutrients in cell-free filtrates. It suggests that the inter-speties competition existed in cyanobacteria. The present study is implicated in illustrating dynamics of the cyanobacserial bloom.