AbstractOxygen uptake ofPseudomonas putidawas employed for the rapid screening of toxicants in water. Oxygen uptake was measured over a 10 minute period at constant temperature by means of a biological oxygen monitoring system connected to a potentiometric recorder. Sublethal concentrations of selected chemicals inhibited or stimulated oxygen uptake. AP. putidagrowth inhibition test proved more sensitive to most individual chemicals than the oxygen uptake assay, but yields results only after some 7 hours. Industrial effluent in low concentrations and domestic wastewater, stimulated both oxygen uptake and growth ofP. putida.Higher concentrations of industrial effluent and a laboratory mixture of selected toxicants inhibited both growth and oxygen uptake at equal levels of sensitivity. Sensitivity compared favourably with that of other rapid bioassays, and various toxicants were detected at concentrations well below those allowed in industrial effluents. Since theP. putidaoxygen uptake assay is relatively sensitive, simple, economical and exceptionally fast, it should prove useful for the rapid testing of many wastewaters and even excessive contamination of drinking‐water supplies while it may serve a valuable purpose as part of a battery of bioassays containing slower test