Abstract:Studies of the genus Phaeocystis, a most widespread marine Prymnesiophyceae, are reviewed. The taxonomy of this genus remains unclear so far. Taxonomic criteria of species are based on initial colony shapes, geographic distribution, free-living cell characteristics as well as chemical products, ploidy, genome size, 18rRNA and so on. Based on all of these taxonomic characters, Phaeocystis has been divided into four definite species at present: P. scrobiculata, P. antarctica, P. globosa and P. pouchetii. The first has only free-living cells and forming no colony，while the rest are of co lony-forming species with not only single cell but also co lonies composed of thousands of cells em bed ded in one mucilaginous matrix occasionally reached several mm in diameter．Phaeocystis has a complex heteromorphic life cycle exhibiting alternation between two different morphological forms：free-living cells and colonies．The unicellular stage involves several types：non-motile．flagellates， microzoospores and possibly macrozoospores ．However，their respective function within Phaeocystis life cycle and，in particular，their involvement in colony formation are not completely.