ABSTRACT: Analytical flow cytometry (AFC) and denaturing gradient gel electrophoresis (DGGE) were used to investigate the diversity and dynamics of viruses, bacteria and phytoplankton communities in a hypereutrophic freshwater lake. Samples were taken from different depths throughout the water column over an annual cycle. Priest Pot is a small lake in the Lake District in NW England (UK), and has been well characterised in previous studies; however, little is known of the diversity and dynamics of the virus community. Virus abundance was shown to change over both spatial and temporal scales, and appeared to be closely linked to other biotic and abiotic parameters. The highest virus concentrations occurred in the deepest part of the lake at a depth of 3.2 m, in the anoxic layer, at the same time as a peak of abundance of green sulphur bacteria. Sequence analysis of a DGGE band that occurred at the same time suggested a bacterium similar to Chlorobium spp., a green sulphur bacterium, comprised part of this bacterial bloom.