Phytoplankton species composition and primary production were studied in the Atlantic Sector of the Southern Ocean in early austral summer 1995/1996. Results from photosynthesis-irradiance experiments (P vs E curves) were used to examine photosynthetic adaptation in this part of the ocean. The study area comprised 3 different provinces: the Antarctic Polar Front (APF), the Antarctic Circumpolar Current (ACC) beyond the influence of frontal systems, and the marginal ice zone (MIZ). Phytoplankton composition derived from HPLC data, P*(m) (maximum biomass-specific photosynthetic production rate) values and areal daily primary production (ADP) rates showed different features for these zones. The central 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 exceeding 900 mg C m-2 d-1. At the fringes of the APF core, phytoplankton were smaller, diatom abundance decreased and dinoflagellates, prymnesiophytes and chrysophytes became more important within the community. Chlorophyll a concentrations and ADP rates were low and comparable to values for the ACC outside the front: <0.5 mg m-3 and <300 mg C m-2 d-1 respectively. Beyond the frontal systems, P*(m) values from the 1% light depth were significantly higher than at the surface. There was also a bloom of large phytoplankton species within the MIZ, dominated in contrast by Phaeocystis spp.; this province was characterized by ADP rates of 558 mg C m-2 d-1. Vertical mixing processes, temperature, silicate concentrations and zooplankton grazing seem to be the factors controlling production and growth of phytoplankton at this time.