Numerous studies confirm poly-beta-hydroxybutyrate (PHB) synthesis by activated sludge under anoxic/aerobic conditions at high COD/N ratio in wastewater. In the presented experiment poly-beta-hydroxybutyrate (PHB) storage and degradation were observed in activated sludge at low COD/N ratio ? 3 in wastes. The researches were carried out in a single-stage system with activated sludge under constant oxygen supplied conditions. Readily biodegradable fraction increased in municipal wastewater through the addition of 0,2 g/dm3 acetate. Moreover, ammonium nitrogen was added to the wastes on the level of 50 mgNH4+/dm3. During the reaction time, organic carbon compounds oxidation by activated sludge and intracellular poly-beta-hydroxybutyrate (PHB) accumulation were observed. Under aerobic conditions and at low COD/N ratio ? 3 in wastewater, activated sludge used accumulated polymer as endogenous carbon source for denitrification. The obtained results show that poly-beta-hydroxybutyrate (PHB) synthesis is possible under fully aerobic conditions and at low COD/N ratio.