<jats:p>Environmental contextRice is a major human dietary source of arsenic. We identified a novel organoarsenic species, arsinothricin, produced by a bacterium in the rice rhizosphere. This result suggests diverse biochemical dynamics and microbial biodiversity of arsenic metabolism in the rice rhizosphere. AbstractMethylated arsenic compounds in rice grains originate from the action of soil bacteria in the rice rhizosphere. Here, we investigated the chemical structures of arsenic compounds produced by a bacterium, Burkholderia gladioli strain GSRB05, in the rice rhizosphere. When cultured in liquid R2A medium containing arsenite (AsIII), strain GSRB05 produced two unknown novel arsenic compounds that were later identified as arsinothricin (AST, 2-amino-4-(hydroxymethylarsinoyl)butanoic acid), an arsenic mimetic of the herbicide phosphinothricin, and a probable hydroxyl precursor of AST, termed AST-OH (2-amino-4-(dihydroxyarsonoyl)butanoic acid). The chemical structure of AST was determined by means of liquid chromatography–high-resolution tandem mass spectrometry and NMR analyses, whereas that of AST-OH was estimated by means of ultra-high-performance liquid chromatography–tandem mass spectrometry. Time-dependent AsIII transformation by strain GSRB05 showed that AST was produced after AST-OH. Compared with AsIII, AST showed higher absorption by, and was more toxic to, Escherichia coli DH5α cells in M9 minimal medium, which lacks amino acids. These findings have implications for the environmental transfer of arsenic, and human health consequences in terms of our dietary burden of arsenic.