Despite significant research dedicated to the marine genus Phaeocystis, which forms large blooms in the coastal waters of the Southern North Sea, some aspects of the taxonomy and biology of this species still suffer from a sketchy knowledge. It is currently admitted that P. globosa is the species that blooms in the Southern North Sea. This has however, never been confirmed by SSU rDNA sequencing which constitutes nowadays, together with the morphology of the haploid flagellate, a reliable taxonomic criterion to distinguish between Phaeocystis species. Also, although the fine morphology of the haploid scaly flagellate is Well known, there is a lack of comparable and harmonized description of the other cell types, i.e. colonial cells and diploid flagellates, previously identified within the Phaeocystis life cycle. In this study, we used SSU rDNA sequencing, light and electron scanning microscopy and flow cytometry to identify and characterize three cell types produced in controlled and reproducible manner from two strains of Phaeocystis isolated from the Belgian coastal zone. Our morphometry and sequencing data confirm unambiguously that P. globosa is the species that blooms in the Southern North Sea, but suggest in addition that both strains are representative of the original P. globosa Scherffel. Our study compares, for the first time since the species description, the fine morphometry and ploidy features of diploid colonial cells, diploid and haploid flagellates originating from same strains, providing therefore unambiguous identification criteria for distinguishing them from each other. The diploid stage, colonial or flagellated cell, is thus characterized by a naked surface, has a size range nearly twice that of the haploid flagellate and do not produce the chitinous filaments specific of the haploid stage. Colonial cells lack flagella and haptonema but possess on their apical side two appendages, which elongate to form the flagella of the diploid flagellate. (C) 2012 Elsevier B.V. All rights reserved.