Arsenic contamination of drinking water is a problem prevalent throughout the world, and it has become more prevalent in the U.S. following a recent reduction in the maximum contaminant limit (MCL) to 10 µg/L. Because As(V) is typically easier to remove than is As(III), arsenic is typically chemically oxidized using free chlorine or permanganate. However, the potential for exploiting biological arsenic oxidation for water treatment is slowly being realized. In this study, microorganisms were isolated from water and filter media collected from a Greene County, Ohio drinking water treatment plant and from bench-scale filters seeded with filter media from the Greene County plant. These isolates were screened for arsenic-oxidation capability and resistance to arsenic. Molecular techniques, including 16S ribosomal RNA gene sequencing and terminal restriction fragment length polymorphism (T-RFLP) analysis, were used to phylogenetically identify and fingerprint the isolates, respectively. The results demonstrated the presence of arsenic-oxidizing bacteria in the full-scale and bench-scale samples. Microbial arsenic oxidizers could be an attractive alternative to the chemical oxidants that are traditionally used in arsenic treatment. Includes 10 references.