Summary. During the autumn of 1987 a survey was carried out in the Atlantic sector of the Southern Ocean in order to study phytoplankton community structure in relation to hydrological features. The positions of the boundary zones, determined by means of hydrological and chemical properties (especially silicic acid) match with previous studies. The phytoplankton community structure was studied by means of algal pigment fingerprints. A cluster analysis of the main phytoplankton pigments revealed the presence of four distinctive phytoplankton communities in the area. In three cluster groups phytoflagellate pigments of different taxonomical groups were found which showed different relative abundance between the cluster groups. In between the Polar Front Zone and the Weddell Scotia Confluence a fourth group was found which was rich in diatoms as compared to the other cluster groups. High concentrations of the fucoxanthin related 19'-hexanoyloxyfucoxanthin indicated the relative importance of Prymnesiophyceae during fall in this area. The relative contribution of each taxonomical group to total phytoplankton biomass was estimated by using specific pigment ratios. This calculation showed that in this time of the year phytoflagellate biomass (especially Prymnesiophytes) surpasses diatom biomass.