The abundance of both heterotrophic nanoflagellates (HNAN) and bacterioplankton in a large (9km2) ultraoligotrophic Antarctic lake (Crooked Lake) were investigated from December 1992 until November 1993. HNAN abundance peaked in spring, summer and autumn, falling to lowest numbers during the winter. Numbers ranged between 0 and 50.9×104 l−1. Bacterioplankton abundance was highest during the late summer and then fell progressively towards winter and autumn (range 1.19–4.46×106 l−1) In contrast to numbers, mean cell volumes (MCV) of the bacteria reached their highest in spring, and consequently highest bacterial biomass occurred at this time. MCV ranged between 0.052 and 0.224μm3. Bacterial production measurements following the incorporation of [3H] thymidine into DNA and [14C] leucine into protein using a doubling-labelling procedure were undertaken in January, June, August, October and November. Rates varied between 2.8 and 52 ng C l1 h1. On occasions, a significant difference in production rates based on the uptake of leucine and thymidine was observed, suggesting unbalanced growth. Highest rates of production coincided with times of high dissolved organic carbon levels in the water column and lowest production with low levels of DOC. HNAN grazing rates were measured by following the uptake of fluorescently labelled bacteria and averaged 4.8 bacterial cells individual1 day1 at 2 and 4°C. Specific growth rates (h1) ranged around 0.00070–0.00077 in both the field and laboratory, giving doubling times of 37.3 and 41.0 days, respectively. These low rates of grazing and growth indicate that there is no adaptation to low temperatures in these freshwater protists. Based on these data, the gross production efficiency is 24%. HNAN removed between 0.1 and 9.7% of bacterial production per day.