A new amperometric enzyme-linked immunoassay for specific enumeration of Nitrobacter has been developed. This assay uses an electrode made of glassy carbon, on which the immunological reaction is carried out. The method is based on a competitive immunoassay principle, utilising monoclonal primary antibody and alkaline-phosphatase-labelled secondary antibody. The enzyme substrate 5-bromo-4-chloro-3-indolyl phosphate generates an electroactive product which is amperometrically detected. The effects of different parameters on the performance of the sensor have been studied. Quantitative detection of Nitrobacter using the immunosensor has been compared to a previously developed enzyme-linked immunosorbent assay showing compatible results. In addition, the overall assay time can be shortened with this new sensor. A detection limit of approximately 3 x 10(6) Nitrobacter cells/ml was obtained.