Size-selective grazing by Cyclidium sp., isolated as a dominant ciliate bacterivore from the Rimov Reservoir (South Bohemia), was examined using fluorescent labelled bacteria (FLB) produced from natural bacterioplankton or pure bacterial cultures. Sizes of ingested bacteria in food vacuoles were measured directly. Three experimental arrangements were used: (1) Ciliates were grown on the pure culture of Alcaligenes xylosoxidans and fed with various proportions of ’large’ and ’small’ FLB (mean biovolume, 0.377 and 0.202 mu m(3), respectively) prepared from the same bacterial species. Results clearly showed significant selection of larger bacteria. (2) Ciliates were grown on natural bacterioplankton from the reservoir and subsequently fed on FLB prepared from the reservoir bacterioplankton (mean biovolume, 0.065 mu m(3)). Independent of either prey or predator abundance, larger FLB (> 0.100 mu m(3), and especially those > 0.200 mu m(3)) were ingested with much higher frequency than their occurrence in the natural assemblage. (3) Ciliates were grown on the reservoir bacterioplankton and fed by FLB prepared from the culture of Pseudomonas sp. In contrast with previous results, no size selection of the ciliate was found when FLB were different from the bacterial food used to grow the ciliate. Ecological impacts of size-selective bacterivory are suggested.