Abstract: The genus Phaeocystis has been observed to form extensive blooms in the coast water throughout the world, and led to serious natural disasters and economic damage. Phaeocystis has a complex polymorphic life cycle that involves both solitary cells and colony forms. To understand the mechanisms controlling the life cycle of Phaeocystis, we conducted experiments to investigate the responses of growth and colony development to chemical cues released from grazing ciliate and copepod. Exposure to chemical cues derived from copepod Pseudodiaptomus annandalei resulted in the reduced solitary cells abundance, increased colony abundance and colonial diameter significantly. In the presence of chemical cues derived from Eupoltes sp, globosa showed increased solitary cell abundance, decreased colony abundance and size. When confront with chemical cues released by different grazers, different defense strategies can be employed by P. globosa. Larger colony can escape from the grazing by the copepod, and smaller colony can protect colonial cells against ciliate. The induced-defense in Phaeocystis reduced mortality and facilitated the occurrence of bloom, which may significantly affect the structure and function of marine food chain.