Offshore oil-producing platforms are designed for efficient and cost-effective separation of oil from water. However, design features and operating practices may create conditions that promote the proliferation and spread of biocorrosive microorganisms. The microbial communities and their potential for metal corrosion were characterized for three oil production platforms that varied in their oil-water separation processes, fluid recycling practices, and history of microbially influenced corrosion (MIC). Microbial diversity was evaluated by 16S rRNA gene sequencing, and numbers of total bacteria, archaea, and sulfate-reducing bacteria (SRB) were estimated by qPCR. The rates of S-35 sulfate reduction assay (SRA) were measured as a proxy for metal biocorrosion potential. A variety of microorganisms common to oil production facilities were found, but distinct communities were associated with the design of the platform and varied with different locations in the processing stream. Stagnant, lower temperature (< 37 A degrees C) sites in all platforms had more SRB and higher SRA compared to samples from sites with higher temperatures and flow rates. However, high (5 mmol L-1) levels of hydrogen sulfide and high numbers (10(7) mL(-1)) of SRB were found in only one platform. This platform alone contained large separation tanks with long retention times and recycled fluids from stagnant sites to the beginning of the oil separation train, thus promoting distribution of biocorrosive microorganisms. These findings tell us that tracking microbial sulfate-reducing activity and community composition on off-shore oil production platforms can be used to identify operational practices that inadvertently promote the proliferation, distribution, and activity of biocorrosive microorganisms.