Seven species of rotifers representing 6 genera, Epiphanes, Plationus, Asplanchna, Philodina species A, Philodina species B. Platyias, and Brachionus, were exposed to Giardia cysts isolated from the feces of experimentally infected holstein calves. Giardia cysts were prestained with a fluorescein isothiocyanate-conjugated monoclonal antibody and mixed with viable rotifers on 3-well Teflon-coated microscope slides. Organisms were observed with phase-contrast, differential interference contrast, and fluorescence microscopy. Five rotifer species, Epiphanes brachionus, Plationus patulus, Philodina (both A and B), and Platyias quadricornis, ingested varying numbers of cysts, which were retained within the rotifers' bodies throughout the observation period. Rotifer ingestion of Giardia cysts may represent a means of reducing water contamination.