Oxygen consumption by Daphnia magna Straus was investigated as a bioindicator for the presence of 11 common aquatic pollutants: Benzo[b]fluoranthene (BbF), mercury(II) chloride (HgCl(2)), 2-dimethoxyphosphinothioylthio-N-methylacetamide (dimethoate), hexachlorocyclohexane (lindane), 3-(3,4-dichlorophenyl)-1-methoxy-1-methylurea (linuron), 4-chloro-o-tolyloxyacetic acid, bis(tributyltin) oxide (TBTO), carbon tetrachloride (CCl4), tetramethylthiuram disulfide (thiram), 2,4,6-trichlorophenol, and arsenic trioxide (As(2)O(3)). The measurement of oxygen consumption by D. magna was carried out after 15 and 210 min of exposure to the toxicants under controlled laboratory conditions. The technique applied in the present study was found to be useful for the detection of significant changes in oxygen consumption for most toxicants, with the exception of BbF, dimethoate, and CCl4. Significant differences in oxygen consumption were noted, relative to the control treatments, for six of the compounds after only 15 min of exposure. For thiram and TBTO, statistically significant changes in oxygen consumption were recorded after 210 min of exposure. The present results suggest that the measurement of oxygen consumption by D. magna is a useful biomarker and could possibly be used as a biological early warning system for detecting pollutants in the aquatic environment.