A disposable-type microbial sensor was developed for the determination of both the biochemical oxygen demand for nitrification (N-BOD) and inhibiting effects on nitrifying bacteria. The sensor was based on the respiratory activity of nitrifying bacteria immobilized on a miniature oxygen electrode, Typical response times for measuring N-BOD of ammonium standard solutions as well as of wastewater samples were in the range of 6-12 min. A dynamic evaluation of the signals after a measuring time of 120 s also resulted in good reproducibility and sensitivity. A daily profile of a municipal sewage plant was recorded, comparing the biosensor data with two standard methods. For the measurement of nitrification-inhibiting effects a 120-s dynamic signal evaluation was preferred to a steady-state method because of the long recovery times resulting from extended exposure to inhibitors. However. steady-state measurement techniques allowed allylthiourea detection with a ten times higher sensitivity. Because of the advantages of this miniaturized electrode, e.g. short response time, simple measuring procedure and low costs of production, this sensor system is considered to be suitable for commercial application in environmental analysis.