A novel amperometric biosensor for the determination of citric acid in food samples and fermentation broths has been developed. The sensor is composed of citrate lyase (CL, EC 4.1.3.6), oxaloacetate decarboxylase (OAC, EC 4.1.1.3) and pyruvate oxidase (POP, EC 1.2.3.3), co-immobilized in gelatin, and an amperometric transducer. A Clark-type O-2-electrode and a modified Clark-type H2O2-electrode were alternatively used as a transducer. The biosensor covers a linear detection range from 1 mu mol dm(-3) to 1 mmol dm(-3) citrate, with a response time of 2.5 min for the steady state response. The lower detection limit for citrate is 0.5 mu mol dm(-3). The response of the sensor remained constant for 8 days and decreased to 25% after 18 days at 20-23 degrees C. The results obtained from citrate determinations in food samples and fermentation broths agree well with those determined by enzymatic sample anlaysis. The relative standard deviation for citrate determinations with the new biosensor was 2.2% (n = 7).