Abstract: Phaeocystis globosa Scherffel blooms mainly develop as colonies, and thus the transition from solitary cells to colonies is key to understanding P. globosablooms. We tested the hypothesis that a particular solitary cell density is necessary for colony formation, below which colonies will not form. We first cultured solitary cells under different conditions (e.g., temperature, nutrients, aeration, agitation, predator pressure, and initial densities) to identify the solitary cell density at which colonies were first observed. Results showed that solitary cell density at the time of colony appearance varied with different culture conditions but was within an order of magnitude of ~10⁴ cells/mL. Following the culture experiments, a dilution experiment was conducted to determine whether colonies formed when solitary cell density was diluted with f/2 media to less than 10⁴ cells/mL. Results showed that no colonies were formed. However, colonies formed within 24 h when the same inoculum was diluted with a small volume of f/2 media, which allowed the density to increase to greater than 10⁴ cells/mL. Thus, solitary cells of P. globosa are more likely to form colonies at high density.