The chain elongations of trans-octadecenoic acid isomers with double-bond positions delta 4 to delta 15 were studied with rat liver microsomes. The delta 7 and delta 9, trans isomers were converted to trans-9 and trans-11-eicosenoic acids, respectively, at about 40% of the rate of conversion of cis-9-octadecenoic acid to cis-11-eicosenoic acid. The rates of conversion of delta 8, delta 10, delta 11, and delta 12 trans isomers were lower than those of delta 7 and delta 9 trans isomers, but the delta 4, delta 5, delta 6, delta 13, delta 14, and delta 15 trans isomers were not elongated at rates exceeding experimental error.