The metalloid arsenic is the most pervasive environmental toxin and carcinogen, creating severe human health issues world-wide. Massive amounts of methylarsenicals such as monosodium methylarsenate [MSMA or MAs(V)] and aromatic arsenicals such as roxarsone [4-hydroxy-3-nitrophenylarsenate, Rox(V)] have been utilized as herbicides and growth promoters for animal husbandry, respectively. Those pentavalent organoarsenicals gradually degrade into more toxic and mobile inorganic arsenite [As(III)], which leads environmental pollutions. We previously identified a novel aerobic MAs(V) degradation pathway composed of two steps, reduction of MAs(V) to trivalent MAs(III), followed by demethylation of MAs(III) to As(III). These two reactions are catalyzed by different bacteria. We further identified arsI, the gene that encodes the ArsI C-As lyase, a novel MAs(III) demethylase. In this study we show that the legume symbiont Sinorhizobium meliloti Rm1021 reduces both MAs(V) and pentavalent synthetic aromatic arsenicals. S. meliloti first reduces the nitro group of Rox(V) to an amino group, forming 4-hydroxy-3-aminophenylarsenate [HAPAs(V)], and subsequently reduces the arsenate group of HAPAs(V), forming HAPAs(III). A mixed culture of S. meliloti and the soil isolate Streptomyces sp. MD1 with a chromosomal arsI gene could degrade either MAs(V) or Rox(V) into As(III). We propose that aerobic microbial communities can degrade synthetic aromatic arsenicals such as Rox(V) by a multiple-step degradation pathway of sequential reduction and ArsI-catalyzed C-As bond cleavage, similar to the pathway of demethylation of the herbicide MSMA. To identify the gene responsible for reduction of the nitro group of Rox(V), we BLAST-searched the genome of S. meliloti for homologs of nitroreductases and found several candidate genes. Those genes were cloned, heterologously expressed in Escherichia coli and examined for Rox(V) nitroreductase activity. One candidate termed mdaB (modulator of drug activity_B), which encodes an ortholog of bacterial FAD-NADPH-dependent nitroreductases, exhibited roxarsone nitroreductase activity when expressed in cells of E. coli. Purified S. meliloti MdaB reduces Rox(V) to HAPAs(V) in the presence of FAD and NADPH. These results suggest involvement of mdaB in reduction of the nitro group of roxarsone by S. meliloti. Identification of the gene(s) responsible for the second arsenate reduction process [HAPAs[V] to HAPAs(III)] is currently in progress. Supported by NIH grants GM55425 to BPR, and by NSF BIO/MCB Grant 1817962 to MY.