The role of glycogen in the uptake of acetate in anaerobic-aerobic activated sludge without enhanced biological phosphorus removal were investigated. Although the polyphosphate content of the sludge was minimized by lowering the phosphorus feeding concentration, significant acetate uptake and accumulation of polyhydroxyalkanoates (PHAs) were observed in proportion to glycogen consumption under anaerobic conditions. The results of anaerobic inhibition studies, which showed suppressive effects on acetate uptake by a glycolysis inhibitor (iodoacetate) but not by a membrane ATPase inhibitor (N,N’-dicyclohexyl carbodiimide), supported an assumption that glycogen degradation through glycolysis supplies the required ATP and reducing power for PHA synthesis from acetate and consumed glycogen. Under subsequent aerobic conditions, the accumulated PHAs were depleted and the consumed glycogen recovered to the same level as that at the start of the anaerobic phase. Iodoacetate also inhibited the recovery of glycogen under aerobic conditions, suggesting that nearly 50% of the PHAs depleted was used for glycogen synthesis through reversed glycolysis.