From in situ measurements, we characterized the size structure of both phytoplankton productivity and phytoplankton chemotaxonomic groups biomass in contrasting biogeochemical areas of the South Indian Ocean. We highlighted pronounced inter- and intra-zonal variability in these size structures. Across the study area, primary production and biomass was mainly driven by medium- and large-sized diatoms, haptophytes and dinoflagellates.