A lysine sensor for process control of lysine fermentation was developed based on a Clark-type electrode in combination with L-lysine-alpha-oxidase. The enzyme, isolated from Trichoderma viride, was immobilized between a cellulose and a polypropylene foil using a polyurethane resin. Lysine determinations were carried out in a flow-through system as anodic measurements when H2O2 was measured and as cathodic measurements when the consumption of O2 was followed. The sensitivity of the sensor toward other amino acids was determined.