This study evaluates the degradative efficiency of activated biomass collected from a Common Effluent Treatment Plant (CETP) under three different dissolved oxygen (DO) levels, 1, 2 and 4 mg l (1). The change in bacterial diversity with reference to DO levels was also analyzed. Results demonstrate that degradative efficiency was the highest, when the reactor was maintained at 4 mg l (1) DO, but amplicon library analysis showed a greater diversity of bacteria in the reactor maintained at 2 mg l (1) DO. Bacteria belonging to the order Desulfuromonadales, Entomoplasmatales, Pasteurellales, Thermales and Chloroflexales have only been detected in this reactor. Ammonia and nitrate levels in all three reactors indicated efficient nitrification process. Results of this study offer new insights into understanding the performance of activated biomass vis-a-vis microbial diversity and degradative efficiency with reference to DO. This information would be useful in improving the efficiency of any wastewater treatment plant. (C) 2014 Elsevier Ltd. All rights reserved.