The effect of external carbon (acetate) addition on nitrogen and phosphorus removal in a sequencing batch reactor (SBR) employing anaerobic/oxic/anoxic (AOA) regime was investigated. Two SBRs, receiving wastewater with different total organic carbon per nitrogen concentration (TOC/N) ratios (influent TOC/N ratio: 2.2 and 4.5 in Run 1 and Run 2, respectively) were operated by changing the amount of external carbon addition at the beginning of oxic phase. The operations in both reactors revealed that nitrogen and phosphorus removal efficiencies increased with an increase in external carbon addition, reaching more than 90% at an external carbon addition of 45.0 mg-C/L. Moreover, the ratio of anoxic/oxic phosphate uptake rate (PUR), an index reflecting a fraction of denitrifying phosphate-accumulating organisms (DNPAOs) in total PAOs, indicates that nitrogen and phosphorus removal efficiencies were enhanced as the PUR ratio increased. Quantitative fluorescence in situ hybridization (FISH) revealed that external carbon addition facilitated the activity of DNPAOs as well as that of glycogen-accumulating organisms (GAOs). The degree of nitrogen removal by denitrifying GAOs (DNGAOs) seemed to increase with an increase in the dosage of external carbon added.