Disposable-type microbial sensors were prepared for the determination of biochemical oxygen demand (BOD). The yeast, Trichosporon cutaneum, was directly immobilized on the surface of miniature oxygen electrodes using an ultraviolet crosslinking resin (ENT-3400). The oxygen electrodes (15 mm x 2 mm x 0.4 mm) were made on silicon substrates using micromachining techniques. They were Clark-type two-electrode systems with-1021 mV applied to the working electrode. Typical response times of the BOD sensors were in the range of 7-20 min. At 20 degrees C, the sensors' dynamic range was from 0 to 18 mg/1 BOD when a glucose/glutamate BOD standard solution was used. The lower limit of detection was 0.2 mg/l BOD. This value was one order of magnitude lower than that of sensors previously reported. The sensors' operational lifetime of 3 days was satisfactory for a disposable type. The sensors' responses were reproducible to within 8% relative standard deviation. The BOD sensors' were applied to untreated and treated waste waters from industrial effluents and municipal sewage. BOD values determined using these sensors correlated well with those determined by the conventional 5-day BOD determination method.