AbstractA biosensor consisting of immobilized nitrite oxidizing bacteria and an oxygen electrode has been developed for the amperometric determination of NO2 (nitrogen dioxide) gas. The response time for the determination of NO2 was within 3 min. A linear relationship was observed between the current decrease and the NO2 concentration below 255 ppm. The minimum concentration for the determination of NO2 was 0.51 ppm. The current decrease was reproducible within ±4% of the relative error. The selectivity of the microbial sensor for NO2 was satisfactory. The current output of the sensor was almost constant for more than 24 days and 400 assays.