An A/O SBR fed with acetate as the sole carbon source was operated for 183 days to examine the dynamic behavior of Candidatus ‘Accumulibacter phosphatis’ (Accumulibacter) in activated sludge enhancing phosphate release activity due to an increase in pH from 7.0 to 8.2. The result obtained by real-time PCR analysis showed that the copy number of Accumulibacter gradually increased until 89 days after a lag period of around 60 days. After that, polyphosphate-accumulating organisms (PAOs) other than Accumulibacter probably proliferated, considering the dynamic change in phosphate release activity. Consequently, the significant acetate uptake by PAOs likely reduced the proliferation of glycogen-accumulating organisms at high pH.