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An oceanographic cruise was conducted in the Ross Sea (Antarctica) during summer 2001 as part of the Italian National Program for Antarctic Research (PNRA). Extensive areas of pack ice occurred over the Ross Sea, atypical for summer when offshore waters are normally free of ice. The present study focuses on the effects of increased ice coverage on phytoplankton assemblages. Water samples collected at various depths at 72 hydrographical stations in offshore and coastal waters were used to determine size-fractionated phytoplankton biomass as chlorophyll a (chla) concentrations, and HPLC photosynthetic pigments. For the offshore waters, the average chla concentration was 57.8mgm⁻², approximately three times the values recorded under ice-free conditions during summer 1996. In coastal waters, the average chla concentrations were 102 and 206mgm⁻² during January and February, respectively, i.e …