Culture-independent techniques, denaturing gradient gel electrophoresis (DGGE) analysis, and random cloning of 16S rRNA gene sequences amplified from community DNA were used to determine the diversity of microbial communities in gas industry pipelines. Samples obtained from natural gas pipelines were used directly for DNA extraction, inoculated into sulfate-reducing bacterium medium, or used to inoculate a reactor that simulated a natural gas pipeline environment. The variable V2-V3 (average size, 384 bp) and V3-V6 (average size, 648 bp) regions of bacterial and archaeal 16S rRNA genes, respectively, were amplified from genomic DNA isolated from nine natural gas pipeline samples and analyzed. A total of 106 bacterial 16S rDNA sequences were derived from DGGE bands, and these formed three major clusters: beta and gamma subdivisions of Proteobacteria and gram-positive bacteria. The most frequently encountered bacterial species was Comamonas denitrificans, which was not previously reported to be associated with microbial communities found in gas pipelines or with microbially influenced corrosion. The 31 archaeal 16S rDNA sequences obtained in this study were all related to those of methanogens and phylogenetically fall into three clusters: order I, Methanobacteriales; order III, Methanomicrobiales; and order IV, Methanosarcinales. Further microbial ecology studies are needed to better understand the relationship among bacterial and archaeal groups and the involvement of these groups in the process of microbially influenced corrosion in order to develop improved ways of monitoring and controlling microbially influenced corrosion.Corrosion is a leading cause of pipe failure and is a main component of the operating and maintenance costs of gas industry pipelines (3,10,18,23,30,31,(42)(43)(44)54). Quantifying the cost of corrosion generally, and more specifically the cost associated with microbial corrosion, in the gas industry is not easily done and is controversial. Pipeline corrosion was estimated in 1996 to cost the gas industry about $840 million/year (10), and in 2001 it was estimated that the annual cost of all forms of corrosion to the oil and gas industries was $13.4 billion, of which microbially influenced corrosion accounted for about $2 billion (31). While it is well recognized that chemical and microbial mechanisms both contribute to corrosion, it is uncertain what the relative contribution of microbial activity to overall pipe corrosion is. It has been estimated that 40% of all internal pipeline corrosion in the gas industry can be attributed to microbial corrosion (23, 44), but data are needed to confirm or revise this estimate. Basic research to increase our understanding of the microbial species involved in microbial corrosion and their interactions with metal surfaces and with other microorganisms will be the basis for the development of new approaches for the detection, monitoring, and control of microbial corrosion. A thorough knowledge of the causes of microbially influenced corrosion and an effici...