a l-ascorbic acid biosensor based on ascorbate oxidase has been developed. the enzyme was extracted from the mesocarp of cucumber (cucumis sativus) by using 0.05 mol l-1 phosphate buffer, ph 5.8 containing 0.5 mol l-1 nacl. after the dialysis versus phosphate buffer 0.05 mol l-1 ph 5.8, the enzyme was immobilized onto nylon net through glutaraldehyde covalent bond. the membrane was coupled to an o2 electrode and the yielding reaction monitored by oxygen depletion at -600 mv using flow injection analysis optimized to 0.1 mol l-1 phosphate buffer ph 5.8, as the carrier solution and flow-rate of 0.5 ml min-1. the ascorbic acid calibration curve was linear from 1.2x10-4 to 1.0x10-3 mol l-1. the evaluation of biosensor lifetime leads to 500 injections. commercial pharmaceutical samples were analyzed with the proposed method and the results were compared with those obtained by high-performance liquid chromatography (hplc).