Abstract:Microbial community structure of nitromonas and denitrifying bacteria of six different MBRs which were in normal operating mode were studied by using PCR-DGGE. The nitromonas were very similar in different MBRs of aerobic area, and the similarity values were 78.7%~92.1%. However, different environmental conditions, influent water quality, operating parameters and other facters resulted in different microbial community in different MBRs. In addition, five kinds of nitromonas and three kinds of denitrifying bacteria were identified in MBRs, alpha-proteobacteria, bradyrhizobium and nitrobacter winogradskyi were the dominant nitromonas, alcaligenes, paracoccus denitrificans and rhodococcus were the dominant denitrifying bacteria, aerobic denitrifying bacterias were found in the MBR systems, there might be several modes of nitrification and denitrification in MBR systems.