Acetate-utilizing, sulfate-reducing bacteria of the genus Desulfobacter were enriched from an oilfield seawater-injection system. These bacteria were detected in enrichment culture and verified as Desulfobacter by an oligonucleotide DNA probe that targets a Desulfobacter-specific sequence of ribosomal RNA. These Desulfobacter produced extensive biofilm in culture and exhibited high levels of hydrogenase activity, which suggests a sessile habit and a role in the cathodic depolarization mechanism of microbially influenced corrosion. Phospholipid analyses indicated that one of the oilfield isolates of Desulfobacter lacked the fatty acid 10Me16:0, which has been used as a biomarker for this genus. Desulfobacter were infrequently encountered in the oilfield systems studied, and they constituted a small portion of the total microflora at any sampling location.