Phytoplankton species composition and primary production werestudied in the Atlantic Sector of the Southern Ocean in early austral summer1995/1996. Results from photosynthesis-irradiance experiments (P vs E curves)were used to examine photosynthetic adaptation in this part of the ocean. The studyarea comprised 3 different provinces: the Antarctic Polar Front (APF), the AntarcticCircumpolar Current (ACC) beyond the influence of frontal systems, and themarginal ice zone (MIZ). Phytoplankton composition derived from HPLC data,P*m (maximum biomass-specific photosynthetic production rate) values and arealdaily primary production (ADP) rates showed different features for these zones. Thecentral core of the APF was dominated (60%) by a bloom of large (>20 µm)diatoms (Thalassiothrix spp., Pseudonitzschia cf. lineola and Chaetoceros spp.),equal values for P*m at the surface and 1% light depths and ADP rates exceeding900 mg C m-2 d-1. At the fringes of the APF core, phytoplankton were smaller,diatom abundance decreased and dinoflagellates, prymnesiophytes and chrysophytesbecame more important within the community. Chlorophyll a concentrations andADP rates were low and comparable to values for the ACC outside the front: <0.5mg m-3 and <300 mg C m-2 d-1 respectively. Beyond the frontal systems, P*mvalues from the 1% light depth were significantly higher than at the surface. Therewas also a bloom of large phytoplankton species within the MIZ, dominated incontrast by Phaeocystis spp.; this province was characterised by ADP rates of 558mg C m-2 d-1. Vertical mixing processes, temperature, silicate concentrations andzooplankton grazing seem to be the factors controlling production and growth ofphytoplankton at this time.