The composition and dynamics of sulfate-reducing bacteria (SRB) during the waterflooding process of Daqing Oilfield were investigated in this study. PCR-DGGE analysis indicated that the microbial communities were significantly different in each treatment unit, and the dominant members were mainly close to Clostridium sp., Thauera sp., Hydrogenophoga sp.. Pseudomonas sp., Eubacterium sp. and Arcobacter sp. However, the members belonging to SRB were relatively few and mainly consisted of Desulfovibrio sp. and Desulfovibrio profundus. According to APS (adenosine-5'-phosphosulfate reductase) gene clone library and sequence analysis, it was found that SRB mainly belonged to Proteobacteria and Deltaproteobacteria. In the library, the genus Desulfovibrio of Deltaproteobacteria was about 65.2% while genus Desulfomonas, Desulfomicrobium, Desulfohalobium, Desulfonatronum, Desulfbulbus, Desulforhopalus, Desulfnema and Desulforhopalus were occupied nearly 34.8%. It was also proved that most of SRB in the oil field were enriched in the ground treatment unit and they are not in the liquid of oil well. (C) 2009 Elsevier Ltd. All rights reserved.