Changes in the abundance of the components of the microbial plankton between July 1990 and March 1991 in Crooked Lake, one of the largest and deepest freshwater lakes in Antarctica, are described. Chlorophyll a concentration is low (0.2–0.4g·1–1) and there is no discernable spring increase. The phytoplankton is largely dominated by flagellates. Bacterioplankton exhibits a seasonal pattern of abundance ranging from 1.0 × 108·1–1 in July to 3.25 × 108·1–1 in September. Changes in bacterial abundance probably relate to temperature and grazing by heterotrophic and mixotrophic flagellates. Total flagellated protozoan concentrations ranged between 25–136 × 102·l–1. Autotrophic and heterotrophic flagellate abundances were coupled and peaks in their abundance oscillated with peaks in bacterioplankton concentration. Four species of ciliated protozoa, dominated by oligotrichs, particularly the plastidic Strombidium, inhabit the lake. The plankton is characterised by the presence of floes which act as loci for bacteria, flagellates and amoebae and feeding sites for the ciliates and the two sparce metazoan components of the plankton. Crooked Lake is extremely oligotrophic but nonetheless supports a plankton community with a low species diversity and simple trophodynamics.