Using oligonucleotide probes for Eubacteria and four eubacterial subgroups, we monitored changes in bacterial community composition (BCC) with different degrees of grazing pressure, as follows: unfiltered water (UNF, all bacterivores present); <20 mu m (bacteria, heterotrophic nanoflagellates [HNF], and small ciliates); <5 mu m (bacteria and HNF only); and <1 mu m or <0.8 mu m (bacteria only), incubated in dialysis bags. Experiments were conducted in the Rimov Reservoir (South Bohemia) during the clear-water phase (experiment I), a period of law protistan gazing pressure on reservoir bacterioplankton, and during the late summer phytoplankton peak (experiment II), a period of high protistan grazing pressure. In both experiments, there was a significant shift in BCC in the <5 mu m treatments, which came in the form of increased proportions of alpha subclass of the class Proteobacteria (ALF) and Cytophaga/Flavobacterium group (C/F), corresponding with increased bacterivory of the ungrazed HNF populations. Changes in BCC in other experimental treatments were related to preincubation protistan grazing pressure. In experiment I, bacterioplankton were subjected to negligible protistan bacterivory in the reservoir and did not shaw a change in BCC in the predator-free treatment (<1 mu m), while BCC changed in treatments that yielded increases in protistan bacterivory compared with that in the ambient reservoir water. In experiment II, a significant shift in BCC was induced when bacterioplankton that were subjected to heavy predation pressure in the reservoir were transferred into the predator-free treatment. Treatments that induced small changes in protistan grazing pressure (<20 mu m and UNF in experiment II) did not show significant BCC shifts. Proportions of filamentous bacteria (>4 mu m) increased in treatments that yielded large increases in bacterivory. Filament formation also showed season-specific features; in experiment I, all filaments belonged into ALF, whereas in experiment II, most of them hybridized with the C/F probe. We conclude that the sudden shifts that violate the established balance between bacterial production and the protist-induced bacterial mortality led to the significant shifts in cell morphology and BCC.