Cell decay is an important part of microbiological endogenous processes, which consists of cell death (reduction in the amount of active bacteria) and activity decay (reduction in the specific activity of active bacteria). By means of measuring maximal anaerobic volatile fatty acid (VFA) uptake rates (VFAUR), analyzing 16S rRNA with fluorescence in-situ hybridization (FISH) and observing membrane integrity by live/dead staining, the aerobic decay characteristics of glycogen-accumulating organisms (GAOs) in an enriched GAOs sequencing batch reactor (SBR) system were investigated. It was experimentally identified that a highly enriched culture of GAOs (94%) was obtained by maintaining the temperature at 30 degrees C in the SBR and a high m (COD): m (P) at 100 in the feed. The experimental results and calculations revealed that the decay and death rates of GAOs were 0.132 d(-1) and 0.034 d(-1) respectively, which demonstrated that cell death and activity decay accounted for respective 26% and 74% of the total GAOs cell decay. For this reason, cell death was only a minor factor causing the cell decay of GAOs, and activity decay was mostly responsible for this process.