AbstractThe biosynthesis of cyclopropane and cyclopropene fatty acids has been investigated in immature seeds, leaves and callus tissue cultures of several species of Malvaceae. Chemical characterization of labeled cyclopropane and cyclopropene fatty acids obtained from incubations withl‐[14CH3] methionine confirmed that the ring methylene group was derived from the methyl group of methionine. The variation with time in the distribution of radioactivity in the products of incubations with [14CH3] methionine and [2‐14C] acetate suggested that the pathway involved initial formation of dihydrosterculic acid from oleic acid with subsequent desaturation to sterculic acid and α‐oxidation to malvalic and dihydromalvalic acids. Direct evidence in favor of this pathway was provided by the conversion of [1‐14C] oleic acid to dihydrosterculic and sterculic acids and by the desaturation of [1‐14C] dihydrosterculic acid to sterculic acid, the first time that these processes have been demonstrated in higher plants. No conversion of [1‐14C] stearolic acid to sterculic acid could be obtained under the same conditions. The presence of an active fatty acid α‐oxidation system was demonstrated in the callus cultures.