Different stages of the life cycle of Phaeocystis antarctica are described by light and electron microscopy on the basis of cultured material isolated from the Atlantic Sector of the Antarctic Circumpolar Convergence, relatively close to the type locality. Species identity was confirmed molecularly on the basis of small-subunit ribosomal DNA. Colonies (up to 2000 mu m in diameter) are surrounded by a rather thick membrane and are generally spherical, in some cases presenting a conical protrusion with a pointed end at one side. Colonial cells (4-6 mu m) have a rounded or slightly quadrangular outline and possess two chloroplasts. In small and medium-sized colonies, cells may be densely packed under the peripheral colony skin; otherwise they are more or less evenly spaced. Nonmotile cells within or outside the colonies often show one to three short protrusions emerging from a depression between the chloroplast edges. These protrusions may develop into two flagella and one haptonema even within intact colonies. Flagellates of different size (3.5-6.5 mu m) are found in the cultures. Their cell body is rounded, elliptical, or pear-shaped, and has a deep, saddle-shaped groove that is dorsoventrally oriented and flanked by the apical edges of the chloroplasts. Two markedly unequal flagella (< 12 and < 18 mu m, respectively) and a straight and relatively long (< 4.5 mu m) haptonema emerge from the apical groove. Flagellates with scales and five-filament structures were found only in one transmission electron microscope preparation. Scales are extremely thin and of two different sizes, with no clear pattern visible in the material observed. Overall, both flagellates and colonies of P. antarctica present some distinctive characteristics that are discussed in comparison with the other known species in the genus and in the frame of a hypothetical life cycle of the species.