An enzyme biosensor was constructed using a plate platinum electrode and immobilized xanthine oxidase (XOD).Only a very small quantity of enzyme was chemically immobilized on a special silk net.Hydrogen peroxide released during the enzymatic reaction was detected by the electrode at+0.65 V (vs.Ag/AgCl).The electrode was very sensitive to hypoxanthine and its detection limit was 1×10-7 mol/L.When it was applied to the determination of fish freshness,the results agreed well with those obtained by traditional methods-determination of total volatile basic nitrogen (TVB-N) and microbial count.A range for estimating the freshness of river fish was suggested.