We examined changes in the phylogenetic structure and leucine incorporation of bacterioplankton assemblages exposed to variable levels of available resources and bacterivory in a freshwater reservoir. Fluorescence in situ hybridization (FISH) combined with microautoradiography was used to determine bacterial phylogenetic affiliations and the proportions of cells incorporating leucine. Grazing-free and grazing-enhanced treatments were incubated in both P-limited and P-rich areas of the reservoir using dialysis bags, and bottle treatments with and without P enrichment. beta-Proteobacteria, mainly the R-BT065 cluster (subcluster of Rhodoferax sp. BAL47), became the dominant leucine-active bacteria in all treatments. Proportions of active cells belonging to R-BT065 phylotypes considerably increased under conditions of enhanced protistan grazing and phosphorus availability. In contrast, proportions of cells from the Sphingobacteria/Flavobacteria group usually decreased and exhibited very low rates of leucine uptake. gamma-Proteobacteria was the least abundant but nevertheless a highly active part of the assemblage. Phosphorus addition supported incorporation of leucine by all bacterial groups. Although highly vulnerable to predation, only R-BT065 cells exhibited marked increases in both relative abundance within the bacterial community and in proportions of active cells. The phylogenetic composition and leucine-uptake of bacterioplankton was more markedly affected by resource availability than by grazing pressure, which only had a strong effect on the dynamics of R-BT065 bacteria within the P-limited area of the reservoir.