Large spherical sulfur bacteria, 180–375 µm in diameter, were found regularly and in abundance in surface sediments collected from hydrocarbon seeps (water depth 525–640 m) in the Gulf of Mexico. These bacteria were characterized by a thin ‘shell’ of sulfur globule-filled cytoplasm that surrounded a central vacuole (roughly 80% of biovolume) containing high concentrations of nitrate (average 460 mM). Approximately 800 base pairs of 16S rRNA gene sequence data, linked to this bacterium by fluorescent in situ hybridization, showed 99% identity with Thiomargarita namibiensis, previously described only from sediments collected off the coast of Namibia (Western Africa). Unlike T. namibiensis, where cells form a linear chain within a common sheath, the Gulf of Mexico strain occurred as single cells and clusters of two, four and eight cells, which were clearly the product of division in one to three planes. In sediment cores maintained at 4°C, which undoubtedly experienced a diminishing flux of hydrogen sulfide over time, the Thiomargarita-like bacterium remained viable for up to 2 years. During that long period, each cell appeared to undergo (as judged by change in biovolume) one to three reductive divisions, perhaps as a dispersal strategy in the face of diminished availability of its putative electron donor.