A new amperometric biosensor was developed for determining hypoxanthine in fish meat. Xanthine oxidase with pyrrole and polyvinylsulphonate was immobilized on the surface of a platinum electrode by electropolymerization. The determination of xanthine-hypoxanthine was performed by means of oxidation of uric acid liberated during the enzyme reaction on the surface of the enzyme electrode at +0.30V (SCE). The effects of pH, substrate concentration, and temperature on the response of the xanthine-hypoxanthine biosensor were investigated. The linear working range of the enzyme electrode was 1.0 x 10(-7) -1.0 x 10(-3) M of the hypoxanthine concentration, and the detection limit was 1.0 x 10(-7) M. The apparent K-m(app) and I-max of the immobilized xanthine oxidase were found to be 0.0154 mM and 1.203 mu A/mM, respectively. The best pH and temperature value for xanthine oxidase were selected as 7.75 and 25 degrees C, respectively. The sensor was used for the determination of hypoxhantine in fish meat. Results show that the fish degraded very rapidly after seven days and the hypoxanthine amount was found to increase over days of storage.