The occurrence of denitrifying phosphate accumulating organisms (DNPAOs) and the contribution of DNPAOs to biological nutrient removal performance were investigated in a bench-scale A(2)O system. Moreover, the effect of excessive aeration on biological phosphorus removal (BPR) was studied. The experimental results suggested that anoxic phosphorus uptake could occur in the A 20 process, as compared with the conventional A 20 process. Good COD, phosphorus, ammonia nitrogen and total nitrogen removal efficiency (92.3%, 95.5%, 96% and 79.5% respectively) could be achieved. Furthermore, sludge analysis demonstrated that the ratio of anoxic P uptake rate to aerobic P uptake rate reached 69% in such a system and nitrate concentration in the anoxic phase and different organic substrate introduced into the anaerobic phase had significant effects on the anoxic P uptake. It was also found that if the air supply was not adjusted properly wider good anoxic P uptake conditions, excessive aeration of activated sludge at the end of aerobic zone could lead to the deterioration of BPR.