We conducted a transplant experiment to elucidate the effects of different levels of grazing pressure, nutrient availability, especially phosphorus, and the impact of viruses on the changes in the structure of bacterioplankton assemblage in a meso-eutrophic reservoir. A sample taken from the nutrient-rich inflow part of the reservoir was size-fractionated and incubated in dialysis bags in both inflow and dam area. The structure of bacteria] assemblage was examined by fluorescence in situ hybridization using oligonucleotide probes with different levels of specificity. In terms of the relative proportions of different bacterial groups, we found very few significant changes in the bacterioplankton composition after transplanting the treatments to the nutrient-poor dam area. However, we observed marked shifts in morphology and biomass towards the development of filaments, flocs and ‘‘vibrio-like{’’} morphotypes of selected probe-defined groups of bacteria induced by increased grazing pressure. Despite the very high abundances of viruses in all the treatments, their effects on bacterioplankton were rather negligible. (c) 2004 Federation of European Microbiological Societies. Published by Elsevier B.V. All rights reserved.