Strain HNR, isolated from a Membrane Bioreactor (MBR), demonstrates a surprising ability to convert ammonium to nitrogen gas under aerobic conditions while growing heterotrophically. On the basis of phylogenetic analysis of the 16S rRNA gene sequence, strain HNR was related to Acinetobacter calcoaceticus (98.9% identity). Nitrogen balance during heterotrophic growth with 120 mg/l of NH(4)(+)-N showed that 40.2% of NH(4)(+)-N was in the form of N(2) and 52.1% was found in biomass. Only a trace production was either nitrite or nitrate. Further tests demonstrated that nitrite and nitrate were not reduced by strain HNR under aerobic conditions. Neither nitrate reductase (NR) nor nitrite reductase (NiR) activity was detectable in the aerobic reaction mixtures. However, a 0.051 U activity of hydroxylamine oxidase (HAO) was observed. The nitrogen removal was speculated to be via a hydroxylamine intermediate instead of nitrite, which was different from the conventional nitrogen removal pathway. (C) 2010 Elsevier Ltd. All rights reserved.