[Objective] To clarify the ability of Phyllobacterium myrsinacearum RC6b and its mutant strains to degrade diphenylarsinic acid (DPAA). [Methods] The conditions for the co-metabolic degradation of DPAA were optimized with P. myrsinacearum RC6b as the original strain and with sucrose, glucose and sodium acetate as different external carbon sources. Strain RC6b was chemically mutated using treatment with N-methyl-N′-nitro-N-nitrosoguanidine (NTG). The DPAA degradation rate was compared before and after chemical mutagenesis, and the metabolite of DPAA by RC6b mutants was also analyzed. [Results] The DPAA degradation rate was <2% after 28 days of incubation when using DPAA as the sole carbon source. After addition of sucrose, glucose and sodium acetate as external carbon sources the DPAA degradation rate increased significantly, by 14.08%, 15.21% and 15.05%, respectively. Three mutant strains of strain RC6b were obtained using 250 μg/mL NTG as chemical mutagen. All three RC6b mutants showed significantly higher DPAA degradation rates than did the original strain. Mutant strain N-RC6b2 showed the highest DPAA degradation rate of 36.71% after 28 days of incubation when using DPAA as the sole carbon source. The main degradation metabolite of DPAA by strain N-RC6b2 was identified as monohydroxylated DPAA. [Conclusion] The original RC6b strain grew only sparingly using DPAA as the sole carbon source. Addition of co-metabolic carbon sources such as sucrose, glucose and sodium acetate significantly enhanced the biodegradation of DPAA. The chemical mutagen NTG further enhanced the biodegradation of DPAA, with monohydroxylated DPAA as the main intermediate product.