The fatty acid profiles of 74 strains of mycolic acid-containing coryneform bacteria were examined by gas-liquid chromatography. All of the strains contained major amounts of straight-chain and monounsaturated fatty acids although some also possessed substantial amounts of 10-methyloctadecanoic acid. Iso- and anteiso-branched acids were not present. Five distinct fatty acid patterns were evident: (i) Corynebacterium diphtheriae, C. pseudotuberculosis and 'C. ulcerans' strains contained major amounts of hexadecanoic and hexadecenoic acids; (ii) C. glutamicum, C. xerosis and related saprophytic and animal-associated strains, predominantly hexadecanoic and octadecenoic acids; (iii) C. bovis, major amounts of octadecenoic and 10-methyloctadecanoic acids; (iv) 'C. mycetoides', significant amounts of heptadecanoic acid as well as hexadecanoic and octadecenoic acids; and (v) strains related to Rhodococcus possessed significant quantities of 10-methyloctadecanoic acid in addition to straight-chain and monounsaturated acids.