Pesticide residues in food pose a major health risk to consumers and currently there is no commercially available on-site detection method. Acetylcholinesterase based amperometric biosensors have the potential to offer a cheap, sensitive, easy to use and portable detection method for a widely used class of pesticides i.e. organophosphorus pesticides. They work on the principle that the enzyme Acetylcholinesterase is inhibited by organophosphorus pesticides. The challenges in effectively making these biosensors are enzyme immobilization on an electrode, stability under storage and to some extent, reproducibility. In this project Zinc Oxide obtained by the sol gel route was used to immobilize the enzyme on electrodes. The conditions of pH and sol dilution that gave optimum biosensor performance as well as long term storage stability were determined. A calibration curve was then obtained for the detection of the model pesticide Paraoxon. Paraoxon detection in the range 0.035 to 1.38 ppm was shown. This is relevant because current EPA pesticide tolerances in the US are in the 0.1 to 1 ppm range. Lower detection limit is possible with the biosensor.