A screen-printed phosphate biosensor based on immobilized pyruvate oxidase (PyOD, E. C. 1.2.3.3) has been developed for monitoring phosphate concentrations in a sequencing batch reactor (SBR) system. The enzyme was immobilized by a nafion matrix and covered a poly ( carbamoyl) sulfonate (PCS) hydrogel on a screen-printed electrode. PyOD consumes phosphate in the presence of pyruvate and oxygen and generates hydrogen peroxide (H2O2), carbon dioxide and acetylphosphate. The electroactive H2O2, monitored at + 420 mV vs Ag/AgCl, is generated in proportion to the concentration of phosphate. The sensor has a fast response time ( 2 s) and a short recovery period ( 2 min). The time required for one measurement using this phosphate biosensor was 4 min, which was faster than the time required using a commercial phosphate testing kit ( 10 min). The sensor has a linear range from 7.5 muM to 625 muM phosphate with a detection limit of 3.6 muM. There was good agreement (R-2 = 0.9848) between the commercial phosphate testing kit and the phosphate sensor in measurements of synthetic wastewater in a SBR system. This sensor maintained a high working stability (> 85%) after 12 h of operation and involved a simple operation procedure. It therefore serves as a useful tool for rapid and accurate phosphate measurements in the SBR system and probably for process control.