Ammonia-oxidizing bacterial community structures and abundances in membrane bioreactor (MBR), which was utilized to treat municipal wastewater, were analyzed by polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE), cloning of 16S rDNA fragments and quantitative real-time PCR techniques. DGGE analysis indicated marked change of ammonia-oxidizing bacterial communities' structure mainly happened during day 62 to day 80. The results suggest that the dynamics of AOB community structure are important and necessary for the improving and stable nitrogen degradation and the removal in MBR. Cloning and sequencing analyses of screened clones revealed multitude nitrosifying organisms and denitrifying bacteria. which suggest that they may coexist several modes of nitrification and denitrification in MBR. Real-time PCR results demonstrated that the abundance of AOB was <0.01% of the total bacteria in inoculating activated sludge. After domestication period, AOB 16S rDNA and amoA gene content in 120th day was 20.44 and 14.38 times those of seeding activated sludge. The analyses suggest a correlation between the quantity of AOB populations and nitrification efficiency. (C) 2008 Elsevier B.V. All rights reserved.