<jats:title>Abstract</jats:title><jats:p>The epiphyte <jats:italic>Pseudomonas syringae</jats:italic> pv.<jats:italic> syringae</jats:italic> 22d/93 (Pss22d) produces a toxin that strongly inhibits the growth of its relative, the plant pathogen <jats:italic>P. syringae</jats:italic> pv.<jats:italic> glycinea</jats:italic>. The inhibition can be overcome by supplementing the growth medium with the essential amino acid, <jats:sc>L</jats:sc>‐arginine; this suggests that the toxin acts as an inhibitor of the arginine biosynthesis. The highly polar toxin was purified by bioassay‐guided fractionation using ion‐exchange chromatography and subsequent RP‐HPLC fractionation. The structure of the natural product was identified by HR‐ESI‐MS, HR‐ESI‐MS/MS, and NMR spectroscopy experiments as 3‐methylarginine. This amino acid has previously only been known in nature as a constituent of the peptide lavendomycin from <jats:italic>Streptomyces lavendulae</jats:italic>. Results of experiments in which labeled methionine was fed to Pss22d indicated that the key step in the biosynthesis of 3‐methylarginine is the introduction of the methyl group by a <jats:italic>S</jats:italic>‐adenosylmethionine (SAM)‐dependent methyltransferase. Transposon mutagenesis of Pss22d allowed the responsible SAM‐dependent methyltransferase of the 3‐methylarginine biosynthesis to be identified.