Chemolitho-autotrophic ammonia-oxidizing bacteria are responsible for the rate-limiting step of nitrification in a wide variety of environments,making them important in the global cycling of nitrogen.In order to analyze the mmonia-oxidizing bacteria population structure in red soil enrichment culture,a pair of AOB-specific primers was designed,and total microbial genomic DNA of enrichment culture was extracted by the direct method of PCR-amplification template.An AOB-specific 16S rDNA clone library was subsequently constructed.RFLP analysis of 16S rDNA was conducted using enzymes(HhaⅠand RsaⅠ) to digest 16S rDNA from the library.The randomly selected 35 cloned fragments were finally clustered into 3 different phylotypes,with the most abundant phylotype accounting for 94% of the selected 16S rDNA clones and the other two phylotypes for 3%,respectively.Seven clones from 3 phenotypes were sequenced and the 16S rDNA sequences were compared with those from GenBank.Based on the Blast analysis,a lot of Nitrosomonas-like sequences were found in this library.The result shows that there are ammonia-oxidizing bacteria in the red soil and Nitrosomonas sp.is a dominant bacterium in enrichment condition.