Microbiologically influenced corrosion of steel in anaerobic environments has been attributed to hydrogenotrophic microorganisms. A sludge sample collected from the bottom plate of a crude-oil storage tank was used to inoculate a medium containing iron (Fe 0 ) granules, which was then incubated anaerobically at 37°C under an N 2 -CO 2 atmosphere to enrich for microorganisms capable of using iron as the sole source of electrons. A methanogen, designated strain KA1, was isolated from the enrichment culture. An analysis of its 16S rRNA gene sequence revealed that strain KA1 is a Methanococcus maripaludis strain. Strain KA1 produced methane and oxidized iron much faster than did the type strain of M. maripaludis, strain JJ T , which produced methane at a rate expected from the abiotic H 2 production rate from iron. Scanning electron micrographs of iron coupons that had been immersed in either a KA1 culture, a JJ T culture, or an aseptic medium showed that only coupons from the KA1 culture had corroded substantially, and these were covered with crystalline deposits that consisted mainly of FeCO 3 .Iron (Fe 0 ) is an inexpensive metal and is widely used in many industrial processes and industrial/commercial products. When iron contacts an aqueous electrolyte, it readily corrodes. This happens because, as a result of metallurgical and environmental heterogeneities, the electrolytes are not evenly distributed across the surface of the metal and consequently the electric potential is also unevenly distributed. Therefore, electrons flow within the metal from an area of higher electrical potential (the anode) to an area of lower electrical potential (the cathode). At the anode, iron atoms lose electrons and dissolve into ferrous ions (Fe 2ϩ ), whereas cations or elements dissolved in solution (e.g., H ϩ under anaerobic conditions or O 2 under aerobic conditions) are reduced by electrons at the cathode.The corrosion of structures that contain iron is economically devastating. It has been estimated that in the United States alone, the cost of corrosion is 276 billion dollars annually (17). Iron is corroded not only by physiochemical processes but also by the metabolic activity of microorganisms; this metabolic process is termed microbiologically influenced corrosion (MIC). Some 10% of all corrosion damage may be the result of microbial activity (15), and sulfate-reducing bacteria (SRB) are widely regarded as the causative agents of MIC in anaerobic environments (11,12,18,21). The mechanism by which SRB stimulate iron corrosion may occur via the uptake of electrons at the cathodic surface of iron (cathodic depolarization) in conjunction with sulfate reduction (8e Ϫ ϩ SO 4 2Ϫ ϩ 10H ϩ 3 H 2 S ϩ 4H 2 O) (27), while at the anionic surface, iron atoms are oxidized to ferrous ions (Fe 3 Fe 2ϩ ϩ 2e Ϫ ). In fact, certain SRB use not only hydrogen but also iron as a source of electrons for sulfate reduction (1, 9, 22). Because not all SRB grow as fast in the presence of iron as they do in the presence of hydrogen (9), fast-growing SRB...