An aerobic denitrifier was isolated from activated sludge and the isolate possessed an average removal rate of 5.7 mg NO3 (-)-N l(-1) h(-1) without accumulation of NO2 (-)-N (less than 2.1 mg l(-1)). The average removal efficiency of nitrate was 93.7 % in 24 h, when the dissolved oxygen (DO) concentrations ranged from 3.2 to 17.5 mg l(-1). The activity of both nap (periplasmic nitrate reductase) and nir (nitrite reductase), whose corresponding genes (napA and nirS) were amplified by touchdown PCR, could be responsible for the tolerance of DO concentrations. Other three genes relating to narG, norB and nosZ were noted to involve in isolate strain.