Abstract: Toxic Microcystis blooms have caused extensive ecological damage to aquatic environment.Biological control is recommended to be a potential approach.Chlorella ellipsoidea was screened out for analysis of its inhibiting effect on Microcystis growth.Mixed culture of C.ellipsoidea,a strain isolated from waste water,and Microcystis PCC 7806 was carried out to observe competitive growth of two species.Chlorophyll-a/chlorophyll-b ratio was used as an indicator to estimate percentage of C.ellipsoidea in the mixed culture.Results show that C.ellipsoidea grew faster than Microcystis at either 20 or 30 ℃in BG-11 medium.In water from a Microcystis blooming lake,C.ellipsoidea displayed stronger competitiveness over Microcystis.The same phenomenon was observed under low light intensity.No evidence was found that C.ellipsoidea produced any toxic substance that inhibits Microcystis.To compare two organisms in ability to use nutrients,absorptions of nitrogen and phosphorus were measured.Results show that C.ellipsoidea absorbed more phosphorus obviously than Microcystis in BG-11,although they were the same in nitrogen absorption.When N concentration in BG-11 was reduced to onetenth,a low nitrogen condition,C.ellipsoidea still absorbed more nitrogen and phosphorus than Microcystis,demonstrating that C.ellipsoidea is more capable of absorbing nutrients,thus resulting in the growth inhibition of Microcystis in mixed culture.It is,therefore,feasible to make use of the superiority of C.ellipsoidea in interspecies competition for growth to control Microcystis bloom.