To study the effects of allelochemicals of Arundo donax Linn. on the growth of Microcystis aeruginosa, the allelochemicals were extracted with three solvents (methanol, ethyl acetate and hexane), respectively. Based on the observation of algal morphology and the measurement of algal density and cell size, the results showed the allelochemicals extracted with all the three solvents had inhibition on M. aeruginosa. The appearance time of allelochemical inhibition increased as follows, allelochemicals extracted with methanol < ethyl acetate < hexane. As treatment time extended, M. aeruginosa regrew at low doses of allelochemicals extracted with methanol. Their inhibitory effects at high doses were lower than those extracted with ethyl acetate and hexane. After 2 d and 4 d of treatment, the inhibition ratios of allelochemicals extracted with ethyl acetate and hexane reached almost 100%, respectively. The 50% effective concentrations (EC50, 6 d) of allelochemicals extracted with methanol, ethyl acetate and hexane were 0.17 g x L(-1), 0.05 g x L(-1) and 0.08 g x L(-1), respectively. The allelochemicals extracted with methanol caused cavities in cells, those with ethyl acetate caused cells into cavities, pieces and conglomeration, and those with hexane caused goffers on cells initially with conglomeration later. The allelochemicals extracted with all the three solvents decreased the cell size of M. aeruginosa, among which those extracted with ethyl acetate had the strongest action.