Mixed bacterial cultures isolated from an estuarine system are able to demethylate and oxidize dime thy larsinic acid to arsenate. The measured oxidation rate averaged approximately 1 ng DMA·l-1·day-1, which is similar to the rate of DMA production due to algal reduction and methylation in marine systems. Bacteria, therefore, may be a prime mediator in the cycling of arsenic in surface waters. © 1979.