1 The population density, diversity and productivity of the microbial plankton in an oligotrophic maritime Antarctic lake were studied for a 15-month period between December 1994 and February 1996.2 In the lake, concentrations of nutrients and dissolved organic carbon were uniformly low, temperature varied over a small annual range of 0.1–3 °C, and the surface was ice-covered except during a period of approximately 6 weeks in summer.3 The total of 57 morphotypes of protozoa observed during the study is a higher taxonomic diversity than previously reported from continental Antarctic lakes, but lower than that found in more eutrophic maritime Antarctic lakes. Likewise, planktonic abundance and productivity were lower than has been reported in other lakes on Signy Island, but generally higher than those of lakes on the Antarctic continent.4 There were marked seasonal and interannual variations in planktonic population density.5 Chlorophyll a concentrations ranged from undetectable to 4.2 µg L-1 and the greatest rate of primary productivity measured was 4.5 mg C m-3 h-1. The phytoplankton was dominated by small chlorophytes and chrysophytes, with phototrophic nanoflagellate abundance ranging from 1.1 × 103 to 1.2 × 107 L-1.6 Bacterial densities of 3.6 × 108 to 1.9 × 1010 L-1 were recorded and bacterial productivity reached a peak of 0.36 µg C L-1 h-1. Numbers of heterotrophic nanoflagellates between 5.0 × 104 and 1.8 × 107 L-1, and of ciliates from undetectable to 1.1 × 104 L-1 were observed. Naked amoebae were usually rare, but occasionally reached peaks of up to 1.5 × 103 L-1.