Rapid toxicity assessment is an important goal in aquatic toxicology. Ecologically important processes that can be assessed more rapidly than mortality or fecundity, while maintaining sensitivity to toxicants would be especially useful endpoints. Feeding behavior of rotifers is significant ecologically because of its direct effects on reproduction, population growth, and algal species composition. Techniques were developed to quantify ingestion rate in the freshwater rotifer Brachionus calyciflorus in 1 h, using fluorescently labeled latex spheres. The number of spheres ingested in a fixed feeding interval is directly proportional to the intensity of fluorescence in the gut. Gut fluorescence was quantified in individual females using an image analysis system. The effects of several types of toxicants on ingestion rate are reported and compared to acute and chronic values from whole animal tests.