The evaluation of crevice corrosion of high alloy stainless steels used in offshore applications is of major importance as it is one of the most deleterious forms of localized corrosion which may result in sudden marine corrosion failure. The resistance of UNS S31803 duplex stainless steel to crevice corrosion in natural seawater was evaluated by immersion and electrochemical tests. Artificially creviced specimens were tested before during and after immersion in natural seawater for up to four weeks allowing indigenous marine microorganisms to adhere to the alloy surface and form biofilm. The biofilm community structure dynamics and its influence on the crevice corrosion of duplex stainless steels in seawater were investigated using a combination of crevice corrosion measurements surface inspection and bacterial community profile analysis by 16S rDNA PCR–DGGE and DNA sequencing. Results indicate that our selection approach to evaluate crevice corrosion yields highly accurate and reproducible results. Crevice corrosion was observed only in anodically polarized specimens and the localized attack was further enhanced by the presence of marine biofilms. An explanation to the possible contributions from the identified biofilm populations to this phenomenon is given.