Using transmission electron microscopy (TEM), a population of giant virus-like particles (VLPs) with a head diameter of ca. 405 nm and a flexible ca. 1100 nm long tail was detected in a lake-type reservoir. These giant VLPs were abundant in situ at the start of a survey period (3.3 x 10(4) particles ml(-1)) and increased by 7-fold within 96 h. This VLP population vanished in dialysis bag incubations of 0.8 mu m-filtered reservoir water (free of bacterivorous flagellates) but increased markedly in the enhanced bacterivory treatment, i.e. 5 mu m filtered water. In the latter, incubation, heterotrophic nanoflagellate (HNF) abundance increased approximately 15-fold during the study. A multiple regression analysis using microbial abundances and grazing rates as parameters indicated that 78% of the variability in the abundance of giant VLPs was explained by HNF abundance and grazing rates. Our data support the hypothesis that this virus population infects flagellates. Observation of a presumptive lysing flagellate cell suggests a viral burst size of 15. Estimations of decay and net production rates from dialysis bag incubations indicate that lysis due to giant viruses could cause between 10 to 60% of the mortality of the total flagellate community and, thus, viruses are potentially a significant factor shaping the population dynamics of flagellates in freshwater.