Nitrifiers are more sensitive than heterotrophic bacteria to environmental changes (such as temperature, pH, shock loads as biological oxygen demand or nitrogen, and toxic/inhibiting loads). Moreover, they are slow-growing bacteria. Thus, a careful control of their activity is needed to achieve continuous good effluent quality. To maintain an actively nitrifying population, efficient process control requires monitoring of the toxicity and the ammonium concentration. The latter is commonly measured by on-line ammonium analyzers. A new sensor has been developed to measure both the ammonia nitrogen (NH 4 + -N) content in the mixed liquor and the activity of the nitrifiers. The operating principle of the biosensor is based on a titration method, which measures the protons produced during nitrification. This paper describes the biosensor and the experimental results (estimation of NH 4 + -N concentration and nitrification rate) obtained using activated sludge from wastewater treatment plants receiving hospital and food industry effluents. The influence of changes in operational factors such as pH, temperature, and bicarbonate alkalinity on the measurements has been examined. The potential applications of the system as an on-line biosensor are outlined.