Microbial arsenic methylation is established as a detoxification process under aerobic conditions (converting arsenite to monomethylated arsenate) 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 paddy soil-derived anaerobic arsenic methylator, Paraclostridium bifermentans strain EML to gain insights into this process. Strain EML was inoculated into a series of media involving systematic dilutions of Reinforced Clostridial Broth (RCB) with 25 μM arsenite to assess the impact of growth substrate on arsenic methylation. Concentrations of MMAs(III) and arsM gene transcription were found to be positively correlated with the RCB dilution, suggesting that substrate limitation enhances arsM gene expression and associated anaerobic arsenic methylation. Anaerobic co-cultures of strain EML with either wild-type Escherichia coli K-12 MG1655 (WT) or E. coli expressing the MMAs(III)-resistance gene (arsP), ArsP E. coli, evidenced increased MMAs(III) production in the presence of E. coli than its absence and growth inhibition of WT E. coli to a greater extent than ArsP E. coli, presumably due to MMAs(III) produced by strain EML. Our findings point to an ecological role for anaerobic arsenic methylation, providing support for a microbial warfare function for this process.