1. The incorporation of [2-(14)C]mevalonic acid by Chloropseudomonas ethylica strain 2K into chlorobactene was studied. 2. Oxidative degradation of chlorobactene of constant specific radioactivity produced labelled benzenecarboxylic acids and indicated that the benzene ring originates from mevalonic acid. 3. Decarboxylation studies demonstrated a stereospecific methyl migration in the formation of the 1,2,5-trimethylphenyl group of chlorobactene. The migrating methyl group was derived from the C-3' position of mevalonic acid.