Phytoplankton species composition and primary production 'ivere studied in the Atlantic Sector of the Southern Ocean in early austral Summer 1995Summer /1996 Results from photosynthasis-irradiance expenments (P vs E curves) were used to examine photosvnthetic adaptation in this pdrt 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 (411Z). Phytoplankton cornposition denved from HPLC data, P', (maximum biomass-specific photosynthetic production rate) values arid areal daily pnmary production (ADP) rates showed different features for these Tones. The central core of the APF was dominated (60°:7] by a bloom of large (>20 pm) diatoms (Thalassiothrix spp., Pseudonitzschia cf. lineola and Chaetoceros spp.), equal values for P;, at the surface and 1 ''o Light depths and ADP rates exceeding 900 mg C m-2 d-'. At the fringes of the APF core, phytoplankton were smaller, diatom abundance decreased and dinoflagellates, prymnesiophytes and chrysophytes became more important within the rommunity. Chlorophyll a concentrations and ADP rates were low and comparable to values for the A<:C outside the front: ~0 . 5 mg m-"nd 1300 m g C mv2 d-' respectively. Beyond the frontal systems, P',,, values from the I?!, 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 charactensed by ADP rates of 558 mg C m-2 d". Vertical mixing processes. temperature, silicate concentrations and zooplankton grazing seem to be the factors controlling production and growth of phytoplankton at this time.