Bombykol [(10E,12Z)-10,12-hexadecadien-1-ol], the sex pheromone of the silkworm moth (Bombyx mori L.), is biosynthesized from hexadecanoate by three successive steps; i.e., D11desaturation, D10,12-desaturation, and reduction of the acyl group. 11,12-Erythro-and 11,12-threo-[7,8,11,12-D 4 ]hexadecanoic acid (D 4 -16:Acid) were synthesized via the catalytic hydrogenation of the corresponding 7,11-diene compounds with a Wilkinson's catalyst under deuterium gas and applied to the pheromone gland of B. mori. GC-MS analysis of their conversion into bombykol and (Z)-11-hexadecen-1-ol, another component in the pheromone gland, revealed that the D11desaturation proceeded in syn-elimination. A previous experiment using [16,16,16-D 3 ]16:Acid has shown that some cyclopropene fatty acids and the amides inhibit the desaturation steps of bombykol biosynthesis. In order to define the structure-activity relationships, 13,13,14,14, 15,15,16,16,16-D 9 -11-hexadecenoic acid (D 9 -Z11-16:Acid) was synthesized from D 10 -1-butanol; and the effect of cyclopropene compounds on the incorporation of D 9 -Z11-16:Acid into bombykol was examined utilizing a selected ion monitoring technique of GC-MS. The result confirmed that D10,12desaturation was strongly inhibited by 11,12-methylenehexadec-11-enoic acid and the amide, and moderately by 13,14-methylene derivatives, while 7,8-and 9,10-methylene derivatives did not inhibit this desaturation. Arch. Insect