Microorganisms capable of degrading diphenylarsinic acid (DPAA) were enriched from contaminated soil using the soil-charcoal perfusion method. Two novel bacterial strains, L2406 and L2413, that can degrade DPAA in a mineral salt medium supplemented with DPAA as the sole carbon source were isolated. Based on comparative morphology, physiology, and comparison of the 16S rRNA gene sequences, both were presumed to be species closely related to Ensifer adhaerens. As the metabolites, phenylarsonic acid (PAA) was determined by liquid chromatography-mass spectrometry analysis as well as three unknown peaks all of whose molecular weights were estimated to be 278. The increase of m/z = 16 from DPAA in the unknowns suggests monohydroxylation of DPAA at the 2-, 3- and 4-positions. The ability of strains L2406 and L2413 to degrade DPAA was suppressed in iron insufficient conditions, e.g. less than 7.2 mu M iron in the culture medium. These facts strongly suggest the following hypothesis: Monooxygenase works at the initial degradation step of DPAA degradation by the isolates; and direct hydrolysis from DPAA to PAA is not likely to occur. In addition, release of arsenic acid from PAA by strain L2406 was confirmed by liquid chromatography-inductively coupled plasma mass spectrometry. From these results, strain L2406 was considered to be capable of degrading DPAA to arsenic acid via PAA when DPAA was supplied as the sole carbon source.