A sequencing batch reactor under different electron acceptor conditions was operated serially to investigate the selection and dominance mechanisms of denitrifying phosphate-accumulating organisms (DNPAOs) in a biological nutrient removal process. The presence of a small amount of NO3- at the start of the anaerobic phase stimulated the selection of DNPAOs in an anaerobic/aerobic system, and switching O-2 to NO3- as an electron acceptor enhanced the activity of anoxic phosphate uptake.