A heterotrophic nitrifier, strain NR, was isolated from a membrane bioreactor. Strain NR was identified as Alcaligenes faecalis by Auto-Microbic system and16S rRNA gene sequence analysis. A. faecalis strain NR shows a capability of heterotrophic nitrification and N2O and N-2 production as well under the aerobic condition. Further tests demonstrated that neither nitrite nor nitrate could be denitrified aerobically by strain NR. However, when hydroxylamine was used as the sole nitrogen source, nitrogenous gases were detected. With an enzyme assay, a 0.063 U activity of hydroxylamine oxidase was observed, while nitrate reductase and nitrite reductase were undetectable. Thus, nitrogenous gas was speculated to be produced via hydroxylamine. Therefore, two different metabolic pathways might exist in A. faecalis NR. One is heterotrophic nitrification by oxidizing ammonium to nitrite and nitrate. The other is oxidizing ammonium to nitrogenous gas directly via hydroxylamine. (C) 2012 Elsevier Ltd. All rights reserved.