Over the past decade numerous lineages of bacteria have been shown to obtain energy for growth through redox transformations of arsenic. However, phototrophic growth using reduced arsenic as an electron donor has not been described. Here we report the light-dependent oxidation of arsenite to arsenate, coupled with autotrophic growth, by an Ectothiorhodospira-dominated consortium of bacteria from alkaline, hypersaline Mono Lake, California. Pure cultures of the Mono Lake Ectothiorhodospira were not capable of phototrophic arsenite oxidation under the culture conditions tested. Electron micrographs of the culture showed a close association between consortia members, although the specific contribution of the individual bacteria is currently unknown. This report extends the list of compounds known to support anoxygenic photosynthesis and documents a previously unknown pathway in arsenic geochemistry.