WE have screened a large number of R plasmid-bearing Escherichia coli and obtained a few that confer resistance to the organomercurials phenylmercuric acetate (PMA) and methylmercuric chloride (MMA). Resistance to cationic Hg(II) in E. coli, Staphylococcus aureus and Pseudomonas aeruginosa has invariably been associated with plasmids that also mediate resistance to various antibiotics (R plasmids)1-6. The mechanism of mercury resistance, is the enzymatic reduction of Hg(II) to Hg(0), which is volatile2,7,8. Previously, no organomercurial-reducing E. coli strains have been reported. The only organomercurial-reducing strain studied in detail is a PMA-resistant soil pseu-domonad7-9. As organomercurials including PMA are a human health problem10, it will be of interest to see if the mercury(ial) resistance of the bacteria in our alimentary canal influences the fate of ingested mercury(ials).