This study was conducted to obtain a better insight into the metabolic behavior of denitrifying phosphate-accumulating organisms relative to the transformations of relevant intracellular compounds as well as phosphorus and nitrate for enhanced biological phosphorus removal under different combinations of electron acceptor (oxygen or nitrate) and electron donor (acetate). Under anoxic conditions, the amount of polyhydroxybutyrate (PHB) produced per acetate taken up considerably increased with the increasing amount of nitrate reduced whereas the amounts of nitrate reduced and phosphorus released per acetate taken up remained almost constant. However, glycogen utilization occurred during PHB production and then was again observed in response to the initial supplementation of acetate after glycogen accumulation was transiently observed during anoxic phosphorus uptake using nitrate as an electron acceptor. On the other hand, under subsequent aerobic conditions, the additional supplementation of acetate again caused aerobic phosphorus release and PHB production, which showed that PHB production was associated with polyphosphate cleavage regardless of electron acceptor conditions. In contrast to anoxic conditions, glycogen accumulation was observed during PHB production. Based on these observations, the preliminary model for the metabolic behavior of denitrifying phosphate-accumulating organisms was proposed and could well account for the complex transformations of PHB and glycogen together with phosphorus release in the presence of acetate under different electron acceptors.