The structuring role and predation im using in situ enclosure experiments in Rimov Reservoir during the spring of 2000. Seasonal changes in abundance and birth rates of the rotifer species Synchaeta lakowitziana, Polyarthra spp., Keratella cochlearis and Kellicottia longispina were related to predation pressure, i.e. selectivity and cropping rates of C. vicinus. All four rotifer species were found to be consumed by C. vicinus whose predation rates showed a marked succession through the spring period and corresponded to changes in the relative abundance of the pelagic species. The highest predation rates exhibited by C. vicinus were oil Synchaeta lakowitziana and Polyarthra spp., being 11.3 and 6.4 ind Cyclops(-1) day(-1) respectively. The activity of the predator switched among three types of preferred food during the spring period. The most preferred, Synchaeta, was replaced by Polyarthra after Synchaeta became extinct and these species were replaced by Keratella when both soft-bodied forms were not available in sufficient quantities. Loricated species (K. cochlearis and K. longispina) were consumed at a slower rate. We calculated the proportion of rotifer production cropped day(-1) in order to estimate the predation impact of C. vicinus, which ranged from 11.6 to more than 100% over time and rotifer species. This shows that relative predation impact can be high not only oil species most strongly selected, but also on species which are not selected.