The use of the freshwater rotifer Brachionus calyciflorus as a test organism for ecotoxicological studies was examined. Changes in the feeding-, swimming behavior- and demographic characteristics of the rotifers exposed to increasing concentrations of copper (Cu) were studied and the relationships between the different test parameters and their ecological relevance evaluated. Relatively simple short-term bioassay methods were developed to measure the swimming activity (2 hours) and the filtration (5 hours) rate. Life-table experiments were performed to asses the long-term effects of Cu on the rotifers. Significant reductions in the filtration rate, the swimming activity and the demographic parameter r were observed at 20, 12 and 5 mug l-1 of Cu, respectively. The implications and the possible ecotoxicological applications of the results are discussed.