This study investigated the effectiveness of denitrification under aerobic conditions depending on nitrogen forms in synthetic wastewater (ammonium, or ammonium and nitrite). Activated sludge was cultivated in a sequencing batch reactor with municipal wastewater enriched by acetate. Poly-beta-hydroxybutyrate (PHB) was accumulated in activated sludge to 0.35 g PHB g(-1) VSS. Activated sludge, cultivated in such conditions, was used in further experimental series. The duration of each series was 24 h. Two types of synthetic wastewater, with acetate as the carbon source, were used in this study. One type of wastewater contained only ammonium; the second one was enriched also by nitrite. The amount of nitrogen reduced by the microorganisms in the activated sludge was 22.5 mg N(red) l(-1) when ammonium was the only nitrogen source in wastewater, and a 3-fold increase was observed in the presence of two nitrogen sources: ammonium and nitrite. Simultaneous consumption of organic substances in wastewater (external source of electron donors) and intracellularly stored poly-beta-hydroxybutyrate in activated sludge (endogenous carbon source) was revealed. COD consumption to reduce 1 mg N-oxides, in series with wastewater containing ammonium, was 8.4 mg COD. However, in series using wastewater with ammonium and nitrite, a 3-fold decrease in COD/N(red) ratio was observed.