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We estimated the abundance and single-cell activity of bacterial groups in waters off the West Antarctic Peninsula (WAP) using a combination of microautoradiography and fluorescent in situ hybridization (FISH). The abundance of the Ant4D3 subgroup, detected by a new FISH probe, was 10% of the total community and half of the gammaproteobacterial population. The Ant4D3, Polaribacter, and SAR11 subgroups accounted for the majority of the Gammaproteobacteria, Sphingobacteria–Flavobacteria, and Alphaproteobacteria respectively. Approximately 40% of the total microbial community actively incorporated leucine (added at 20 nmol L-1), while a smaller fraction (12-22%) used protein and an amino acid mixture (added at tracer concentrations). The fractions of SAR11, Polaribacter, and Ant4D3 that were active differed from each other and varied among substrates. SAR11 had the largest fraction of active cells incorporating leucine, while Polaribacter dominated the community using protein. The fraction of Ant4D3 using different compounds did not vary, but this group dominated the incorporation of amino acids, and was an abundant and active component of the bacterial community. Bacteria in the WAP region were as active as bacteria in the Mid-Atlantic Bight, even though total bacterial production was lower in the WAP. Though persistently cold (0-1°C) and dominated by different bacterial taxa, the single-cell activity of this summertime Antarctic bacterial community was comparable to that of temperate communities. [ABSTRACT FROM AUTHOR]