Although methylation is regarded as one of the main detoxification pathways for arsenic (As), current knowledge about this process during manure composting remains limited. In this study, two pilotscale compost piles were established to treat manure contaminated with As. An overall accumulation of methylated As occurred during 60 day-composting time. The concentration of monomethylarsonic acid (MMA) increased from 6 to 190 μg kg −1 within 15 days and decreased to 35 μg kg −1 at the end of the maturing phase; while the concentration of dimethylarsinic acid (DMA) continuously increased from 33 to 595 μg kg −1 over the composting time. The arsM gene copies increased gradually from 0.08 × 10 9 to 6.82 × 10 9 copies g −1 dry mass over time and correlated positively to the concentrations of methylated As. 16S rRNA gene sequencing and arsM clone library analysis confirmed the high abundance and diversity of arsM genes. Many of these genes were related to those from known As-methylating microbes, including Streptomyces sp., Amycolatopsis mediterranei and Sphaerobacter thermophiles. These results demonstrated that As methylation during manure composting is significant and, for the first time, established a linkage between As biomethylation and the abundance and diversity of the arsM functional genes in composting manure. The rapid expansion of the poultry and livestock industries in the past decades has generated vast quantities of farming waste, with attendant environmental impacts, notably in farming-intensive countries such as China, India and Brazil 1-3. For instance, ~2.2 billion tons of poultry and livestock manure were generated in China in 2011 alone 4. The manure usually contains a large amount of nutrients, inorganic and organic contaminants, antibiotic resistance genes, and pathogens, most of which are potential sources of pollution and pose risks for the environment 5. Composting is an economical and environmentally friendly approach for reducing and attenuating manure waste 6 , and has been widely used in China and other countries around the world. Currently, the behavior and biotransformation of inorganic and organic pollutants during composting of livestock waste are major research focus. Arsenic (As) is a potent environmental toxin and human carcinogen 7 that is linked to increased risk of bladder, lung, and skin cancers 8 and ranks the top in the list of hazardous substances by US Environmental Protection Agency (EPA). Despite its toxicity, As-based feed additives are commonly used in the poultry and livestock industry to prevent disease, enhance feed efficiency and promote rapid growth 9. Not readily absorbed in animal tissues, almost all the fed As is excreted without attenuation in manure at concentrations up to 300 mg kg −1 9,10. In nature, As exists in inorganic and organic forms such as arsenate [As(V)], arsenite [As(III)], monomethylarsonic acid [MMA], dimethylarsinic acid [DMA], trimethylarsinic acid [TMA] and trimethylarsine oxide [TMAO] with varying biogeochemical behaviors and toxicity 11. Meth...