Marine snow is a ubiquitous feature of the ocean and is considered to be a key mechanism for transporting carbon to the deep ocean.Although not the only source of marine snow, aggregation of phytoplankton is one of its major contributors. Large fluxes of aggregates are typically coupled to primary productivity and are associated with the termination of phytoplankton blooms. Phaeocystis is a cosmopolitan bloom-forming alga that is often recognized as both a nuisance alga and an ecologically important member of the phytoplankton. Phaeocystis has physiological capability to transform between solitary cells and colonial life cycle stages. The predominant form during the bloom is the colonial phase. Aggregations of senescent colonies were also observed occasionally at the end of the bloom. However, there is yet no direct evidence with regard to the development of aggregation of Phaeocystis solitary cells. In this current study we conducted experiments to test the hypothesis that aggregation formation by P. globosa can be initiated by the introduction of grazers (Oxyrrhis marina). Phaeocystis globosa solitary cells were distributed into twelve 250 mL culture flasks at an initial concentration of 4000 cells/mL. O. marina was added into six of the flasks (initial concentration 0.2 cells/mL) that were used as the grazing treatments, and the other six flasks containing P. globosa solitary cells only served as the controls. We also investigate the effect of turbulence on the aggregation formation by P. globosa solitary cells. Our results demonstrated that the aggregations were formed only in the presence of grazers, which clearly suppressed the accumulation of solitary cells. These results strongly suggest that the development of colonies was initiated by grazing. These aggregations also had a very unique structure: cells without flagella closely clumped together to form an approximately spherical structure. Microscopic observations showed that these colonies lacked the mucus envelop that is characteristic of Phaeocystis colonies, and that naked cells were seen clearly on the colony surface. It was unable to tell whether the interior of the colonies was hollow or not, as the cells were packed tightly and the colonies appeared opaque. In our experiments the overwhelmingly high grazer abundances may have greatly impeded the solitary cells to produce polysaccharide material and construct a mucilaginous colony via growth and cell division. Aggregation size and cell numbers per aggregation increased with time, and the development of aggregations effectively protected the cells from grazing. The grazer abundance increased rapidly in the first twelve days, but then it began to fluctuate, coinciding with the time when colonies began to occur. We speculate that the formation of aggregations may represent an energetically inexpensive means to increase the collective size against grazing. Through a combination of life cycle stages and possibly grazer-activated defenses, Phaeocystis appears to have a greatly reduced mortality compared to competing phytoplankton.