Arsenic exists in two oxidation states in water, arsenite [As(III)] and arsenate [As(V)]. As(III) is relatively mobile in water and difficult to remove by arsenic-removal treatment processes. Source waters that contain As(III) must add a strong oxidant such as free chlorine or permanganate to oxidize the arsenic. This article highlights an Ohio treatment plant where natural bacteria in the source water concentrate in filters and oxidize As(III), eliminating the need for a strong oxidant ahead of filtration. Microbial filtration also provides secondary benefits such as reduced chemical- handling issues, maintenance of nitrification capacity in the filters, elimination of nitrification potential in the distribution system, and reduced chlorine demand in the finished water. Microbial treatment of drinking water is not widely accepted in the United States. Results of this research demonstrated that As(III) oxidation and its subsequent removal can occur microbially, offering an alternative, nonchemical approach to As(III) oxidation and a safe and relatively simple means of meeting the arsenic drinking water standard. Includes 43 references, tables, figures.