A simple and sensitive biosensor based on mushroom (Agaricus bisporus) tissue homogenate that contains polyphenol oxidase is described for the determination of thiourea. The mushroom tissue homogenate was immobilized to the sensitive top of a Clark-type oxygen electrode with gelatin and glutaraldehyde. The linear range of the system was 1-20 mu M with a detection limit of 1 mu M at pH 7.5 and 30 degrees C. The repeatability experiments were carried out, and the average value, standard deviation, and variation coefficient were calculated as 9.875 mu M, +/- 0.279, and 2.83%, respectively. Interference studies revealed that the substances tested did not interfere with the biosensor. Finally, the proposed biosensor was applied to the determination of thiourea in fruit juices.