The seed oil of meadowfoam (Limnanthes alba) and other Limnanthes spp. is enriched in the unusual fatty acid ⌬ 5 -eicosenoic acid (20:1⌬ 5 ). This fatty acid has physical and chemical properties that make the seed oil of these plants useful for a number of industrial applications. An expressed sequence tag approach was used to identify cDNAs for enzymes involved in the biosynthesis of 20:1⌬ 5 ). By random sequencing of a library prepared from developing Limnanthes douglasii seeds, a class of cDNAs was identified that encode a homolog of acyl-coenzyme A (CoA) desaturases found in animals, fungi, and cyanobacteria. Expression of a cDNA for the L. douglasii acyl-CoA desaturase homolog in somatic soybean (Glycine max) embryos behind a strong seed-specific promoter resulted in the accumulation of ⌬ 5 -hexadecenoic acid to amounts of 2% to 3% (w/w) of the total fatty acids of single embryos. ⌬ 5 -Octadecenoic acid and 20:1⌬ 5 also composed Ͻ1% (w/w) each of the total fatty acids of these embryos. In addition, cDNAs were identified from the L. douglasii expressed sequence tags that encode a homolog of fatty acid elongase 1 (FAE1), a -ketoacyl-CoA synthase that catalyzes the initial step of very long-chain fatty acid synthesis. Expression of the L. douglassi FAE1 homolog in somatic soybean embryos was accompanied by the accumulation of C 20 and C 22 fatty acids, principally as eicosanoic acid, to amounts of 18% (w/w) of the total fatty acids of single embryos. To partially reconstruct the biosynthetic pathway of 20:1⌬ 5 in transgenic plant tissues, cDNAs for the L. douglasii acyl-CoA desaturase and FAE1 were co-expressed in somatic soybean embryos. In the resulting transgenic embryos, 20:1⌬ 5 and ⌬ 5 -docosenoic acid composed up to 12% of the total fatty acids.