Strain ASV2, an unidentified Gram-negative bacterium newly isolated from activated sludge, was found to utilize arsonoacetate at concentrations up to at least 30 mM as sole carbon and energy source, with essentially quantitative extracellular release of arsenate. Cell-free conversion of arsonoacetate could not be obtained, but resting-cell studies indicated that the carbon-arsenic bond cleavage activity was inducible in the presence of arsonoacetate and was of limited substrate specificity, also breaking down arsonochloroacetate. The inorganic product of the reaction may be arsenite since an inducible arsenite-oxidizing activity was also found in arsonoacetate-metabolizing cells. This is the first report of a micro-organism capable of utilizing a compound containing the carbon-arsenic bond. The results indicate that the ability of bacteria to degrade arsonoacetate is not fortuitous and may be found in environments not previously exposed to organoarsenicals.