Offshore oil production facilities are frequently victims of internal piping corrosion, potentially leading to human and environmental risks and significant economic losses. Microbially influenced corrosion (MIC) is believed to be an important factor in this major problem for the petroleum industry. However, knowledge of the microbial communities and metabolic processes leading to corrosion is still limited. Therefore, the microbial communities from three anaerobic biofilms recovered from the inside of a steel pipe, exhibiting high corrosion rates, iron oxides deposits and substantial amounts of sulfur, which are characteristic of MIC, were analyzed in detail. Bacterial and archaeal community structures were investigated using Automated Ribosomal Intergenic Spacer Analysis (ARISA), multigenic (16S rRNA and functional genes) high throughput Illumina Miseq sequencing and quantitative polymerase chain reaction analysis. The microbial community analysis indicated that bacteria and particularly Desulfovibrio species dominated the biofilm microbial communities. However other bacteria such as Pelobacter, Pseudomonas and Geotoga, as well as various methanogenic archaea, previously detected in oil facilities were also detected. The microbial taxa and functional genes identified suggested that the biofilm communities harbored the potential for a number of different but complementary metabolic processes and that MIC in oil facilities likely involves a range of microbial metabolisms such as sulfate, iron and elemental sulfur reduction. Furthermore, extreme corrosion leading to leakage and exposure of the biofilms to the external environment modify the microbial community structure by promoting the growth of aerobic hydrocarbon degrading organisms.