The effect of Chaoborus predation on natural zooplankton communities was tested in enclosure experiments during June, July and August in the eutrophic Frederiksborg Slotssø, Denmark. Natural densities of Chaoborus larvae in enclosures reduced the abundance of copepod nauplii and Chydorus sphaericus with up to 60% and 80%, respectively, compared with controls. In contrast, Chaoborus was not able to suppress populations of Eudiaptomus graciloides and Daphnia cucullata. Predation was strongest during June and decreased thereafter as Chaoborus pupae emerged. Laboratory experiments showed that a considerable part (20-30% of the population day^-1) of Chaoborus larvae emerged as pupae at 18-20 °C in July and August. Chaoborus larvae occurred in the lake as migratory individuals from May to September with mean population densities of about 100 larvae m^-2. Results from the enclosure experiments suggest that the populations of herbivorous zooplankton in Frederiksborg Slotssø during June, July and August are not controlled by natural populations of Chaoborus larvae.