Optimizing anoxic biological phosphorus removal in the anaerobic-anoxic sequencing batch reactor (A2 SBR) was observed to depend greatly on three parameters: the amount of added nitrate, the sludge retention time (SRT) and the phosphorus/carbon feeding ratio (P/C, wt/wt). The concentration of 120 mg N-NO3 l-1 in the anoxic medium corresponding to 800 mg COD l-1 acetic acid and 60 mg P-PO4 l-1 in the synthetic wastewater, the SRT of 15 days and the P/C feeding ratio of 20/100 were determined as optimal for complete phosphorus removal in the A2 SBR. The acetate uptake, the phosphorus release and the phosphorus removal increased with the P/C feeding ratio and the phosphorus sludge content (Ps). The polyphosphate-accumulating organisms (PAO) were dominant under operation at all P/C feeding ratios tested except 2/100. At a P/C feeding ratio of 20/100, PAO could accumulate a high content of polyphosphate compared with other P/C ratios. In contrast, the P/C ratio of 2/100 caused a decrease in the polyphosphate content in PAO, the deterioration of enhanced biological phosphorus removal (EBPR) activity, and the dominance of glycogen-accumulating organisms (GAO). This study was completed by microscopic observations which revealed three morphological types of PAO. This is the first time that an oval form of PAO could be observed in the A2 SBR.