Microbial arsenic (As) methylation is established as a detoxification process under aerobic conditions (converting arsenite) to monomethylarsonic acid (MMAs(V))) but proposed to be a microbial warfare strategy under anoxic conditions due to the toxicity of its main product monomethylarsonous acid (MMAs(III)). Here, we leveraged a recently isolated anaerobic As methylator, Paraclostridium bifermentans strain EML to gain insights into this process. Strain EML was inoculated into a series of media that consisted of systematic dilutions of Reinforced Clostridial Broth (RCB) supplied with 25 μM arsenite to assess the impact of growth substrate availability on As methylation. Growth curves evidenced the sensitivity of strain EML to arsenite, and As speciation analysis revealed the production of MMAs(III). Concentrations of MMAs(III) were found to be positively corelated with the RCB medium dilution, suggesting that growth substrate availability plays an important role in the regulation of MMAs(III) production. Further, reverse-transcription quantitative PCR evidenced arsM gene transcription increased with RCB dilution. Thus, growth substrate limitation is interpreted to enhance arsM gene expression and the associated anaerobic As methylation. We propose that growth substrate competition between microorganisms may also lead to an increase in As methylation, providing support for a microbial warfare function for this process.