A biosensor using an immobilized bienzyme system was developed to determine the artificial sweetener aspartame in foodstuffs. The bienzyme system, consisting of α-chymotrypsin and alcohol oxidase, was co-immobilized by a covalent cross-linking method with glutaraldehyde on a dissolved oxygen electrode. The optimum operating conditions for the biosensor were pH 6.5–8.5 and 30 °C. A linear relationship was observed between the decrease in dissolved oxygen and the aspartame concentration in the range 0.33–2 mmol l^–1. When the enzyme electrode was stored in dithiothreitol solution, the biosensor was stable for more than 50 d and 450 assays. An amperometric biosensor method was established for the assay of aspartame in commercial products without the need for sample pre-treatment or special reagents. Such a development should facilitate rapid quality control testing in the food industry.