Membrane bioreactor (MBR) operation under low dissolved oxygen (DO) condition can potentially minimize the operational costs without compromising nitrification. In this study, the influence of periodic low DO condition to the overall removal of ammonia (NH3) in a lab-scale hypoxic/oxic (H/O) MBR was investigated. Performance evaluation of activated sludge ammonia oxidizing bacteria (AOB) shows that significant concentration of NH3 is initially oxidized in the hypoxic bioreactor and only trace amount of NH3 can be detected in the effluent. This high NH3 oxidizing activity can be supported by the high ATP value measured in the hypoxic activated sludge and from the following results of batch experiments. Similar potential for NH3 removal was observed in both activated sludge but the rate was higher in hypoxic activated sludge (2.50 mgNH(3)-N/L/h) than in oxic activated sludge (1.39 mgNH(3)-N/L/h). Under alternating hypoxic/oxic condition, simultaneous nitrification and denitrification was also observed. The results strongly support the hypothesis that stable and complete nitrification in H/O MBR is more of the adaptive response of AOB to changing environmental condition rather than the selection of specific AOB lineage. The response occurs within short time period (hydraulic retention time: 3 h and 40 min) and the same mixed AOB community is present in both hypoxic and oxic activated sludge which are related to Nitrosomonas eutropha, Nitrosomonas europaea, and Nitrosomonas oligotropha. The results suggest that periodic low DO through alternating hypoxic/oxic condition significantly contributes to efficient nitrification in H/O MBR as it renders the mixed activated sludge AOB the adaptability to DO limitation. (C) 2014 Elsevier B.V. All rights reserved.