The seasonal changes in the abundance of protozoan and bacterial plankton in a large, coloured, oligotrophic lake. Loch Ness (Scotland), were investigated between August 1991 and January 1993. The coloured water supported only low concentrations of chlorophyll a (<1.6 µgl–1). with the highest values occurring in summer. Mean bacterial abundance ranged between 2.3 x 108 and 7.1 x 108 l–1 in the 100 m water column. Maximum abundance did not correlate with maximum chlorophyll a concentrations, but appeared to be related to the input of allochthonous carbon from the catchment, which in turn was influenced by rainfall levels. Consequently, the highest bacterioplankton concentrations occurred in autumn and winter. The pattern of heterotrophic nanoflagellate abundance tended to follow that for bacteria, with mean concentrations in the top 100 m of the water column of between 12 x 103 and 273 x 103 l–1. Ciliate abundance showed no seasonal trends over the study period and probably mirrored the fluctuating availability of various food resources. Oligotrichs, particularly mixotrophic taxa, were a prominent element of the community throughout the year. Aggregates of detrital material were a regular feature in the plankton. When these occurred, they formed foci for bacteria and nanoflagellates. The evidence suggests that the dynamics of the microbial plankton in Loch Ness may be driven by allochthonous carbon inputs rather than by the more usual dominance of carbon fixed within the system.