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The impact of the organic carbon to nitrogen ratio (chemical oxygen demand (COD)/N) in wastewater and dissolved oxygen (DO) concentration on carbon and nitrogen removal etfficiency,and total bacteria and ammonia-oxidizing bacteria (AOB) communities in activated sludge in constantly aerated sequencing batch reactors (SBRs) was determined.At DO of 0.5 and 1.5 mg O2/L during the aeration phase,the efficiency of ammonia oxidation exceeded 90％,with nitrates as the main product.Nitrification and denitrification achieved under the same operating conditions suggested the simultaneous course of these processes.The most effective nitrogen elimination (above 50％) was obtained at the COD/N ratio of 6.8 and DO of 0.5 mg O2/L.Total bacterial diversity was similar in all experimental series,however,for both COD/N ratios of 6.8 and 0.7,higher values were observed at DO of 0.5 mg O2/L.The diversity and abundance of AOB were higher in the reactors with the COD/N ratio of 0.7 in comparison with the reactors with the COD/N of 6.8.For both COD/N ratios applied,the AOB population was not affected by oxygen concentration.Amplicons with sequences indicating membership of the genus Nitrosospira were the determinants of variable technological conditions.