Oxygen consumption rate is a universal and sensitive biomarker of viability and metabolic status of aerobic organisms. We applied the optical oxygen sensing method to monitor the respiration of individual Daphnia magna (D. magna) and to develop a simple, automated screening assay for the assessment of acute toxicity of large numbers of chemical and environmental samples. D. magna were exposed to the toxicants and effluent samples for 24 h or 48 h and then analysed for changes in respiration relative to untreated controls, using standard microtiter plates and a fluorescence reader. Our assay showed the ability to detect sub-lethal effects of reference toxicants, including potassium dichromate, sodium lauryl sulfate, zinc and cadmium, ease of generation of dose-response curves and EC50 values. The assay showed comparable sensitivity and robustness to the conventional assay, and higher sensitivity when testing industrial effluents. It is therefore well suited for environmental monitoring.