Résumé : Despite numerous investigations, the number and role of morphotypes involved in the life cycle of Phaeocystis species remain under debate. This is partly due to the application of diVerent methodologies such as light, transmission, scanning electron microscopy and Xow cytometry on speciWc samples. This heterogeneity of approaches results in the incomplete morphometric description of the diVerent cell types existing within one species according to relevant criteria and the indetermination of the ploidy level of each observed stage. We review here the diVerent morphotypes observed within each of the six Phaeocystis species recognized up to now. Four diVerent cell types have been observed. In common to all six species is the occurrence of a scaly Xagellate producing star-forming Wlaments (all species except P. jahnii) or not (P. globosa and P. jahnii). In three colony-forming species, P. globosa, P. pouchetii and P. antarctica, three morphotypes are observed: a Xagellate with scales and Wlaments, a colonial cell, and a Xagellate devoid of scales and Wlaments. In the non-colonyforming species, P. scrobiculata and P. cordata, only Xagellates with scales and Wlaments have been observed. While suspected in P. pouchetii and P. antarctica, a haploid–diploid life cycle has only been evidenced for P. globosa. The two main prominent features of this cycle are that sexuality is prevalent in colony bloom formation and termination and that two types of vegetative reproduction exist. The ecological relevance of alternating haploid and diploid stages is not clearly apparent on the basis of existing ecological studies.