On-line measurements of the activity of activated sludge biomass may allow more efficient operation of treatment plants. A novel monitor for activity which can be used on-line follows oxygen evolution from hydrogen peroxide by catalase activity in activated sludge. Results from this monitor were compared with assays of oxygen uptake rate (OUR) by respirometry. Samples from the five activated sludge plants were assayed and showed, as expected, that the mixed liquor suspended solids (MLSS) and volatile suspended solids (VSS) levels were a poor measure of the activity of the sample determined by OUR. However, the activity of the biomass determined by the catalase activity biomass monitor correlated well with the respiration rate. This monitor should provide a simple and robust alternative to OUR measurements in activated sludge plants.