It is generally accepted that a low dissolved oxygen (DO) concentration is more beneficial for achieving partial nitrification than high-DO.In this study,partial nitrification was not established under low-DO conditions in an intermittent aeration reactor for treating domestic wastewater.During the operational period of low-DO conditions (DO:0.3 + 0.14 mg/L),stable complete nitrification was observed.The abundance of Nitrospira-like bacteria,which were the major nitrite-oxidizing bacteria,increased from 1.03 × 106 to 2.64 × 106 cells/mL.At the end of the low-DO period,the batch tests showed that high-DO concentration (1.5,2.0 mg/L) could inhibit nitrite oxidation,and enhance ammonia oxidation.After switching to the high-DO period (1.8 ± 0.32 mg/L),partial nitrification was gradually achieved.Nitrospira decreased from 2.64 × 106 to 8.85 × 105 cells/mL.It was found that suddenly switching to a high-DO condition could inhibit the activity and abundance of Nitrospira-like bacteria,resulting in partial nitrification.