A partial nitrification system was investigated for 471 days under DO varying concentrations for assessing its stability and population dynamics. Within 130 days of operation at feed DO concentration of 1.0 +/- 0.1 mg/L, more than 85% of nitrite was accumulated. Efficiency deteriorated when the feed DO concentration was increased to 4.2 +/- 0.3 mg/L Nitrite accumulation could not be re-established on decreasing feed DO to 1.0 +/- 0.1 mg/L. Even at DO concentration of <0.05 mg/L, nitrate production was observed: a condition termed as anoxic nitrification. NOB was detected in the biomass even under this condition by Fluorescence in-situ hybridization (FISH) analysis. Through 16S rRNA gene sequencing a major fraction of unknown bacterial sequences closely resembling haloalkalophilic bacteria of marine origin were detected. The study indicated that these bacterial species might play a role in anoxic nitrification and that NOB could survive extreme low DO condition. (C) 2010 Elsevier Ltd. All rights reserved.