Abstract: Microcystis bloom has caused many environmental problems, and its mechanism of bloom-formation and adaptation has been noticed wildly. To examine the possible information transfer by chemicals between both algae, the effect of Ochromonas and its culture media filtrate on the growth and physiological traits of Microcystis aeruginosa was investigated in this study. The results indicated that the cell density in Microcystis decreased significantly, while the cell density in Ochromonas increased markedly when Microcystis and Ochromonas were co-cultured at three initial cell density (1﹕4, 1﹕1 and 4﹕1). Moreover, the culture media filtrate ofOchromonas sp. could inhibit the growth, and resulted in an increase of malondialdehyde (MDA) content and catalase (CAT) activity. Additionally, the content of extracellular polysaccharides (EPS) increased markedly in Microcystis when it was inoculated into the culture media filtrate of Ochromonas sp. These results indicated that Ochromonas not only could prey on Microcystis, but also might release some of the chemicals to inhibit the growth and physiological parameters of Microcystis. This suggested that Ochromonas might be used to inhibit the early proliferation of algae as a potential control of algae blooms.