(±)-4-Methylocttinoic acid and its ethyl ester are aggregation pheromones of many rhinoceros beetles of the genus Oryctes and are investigated for the control of these pests by olfactorj trapping. A simple, economical, and high-yield (>50%) synthesis of (±)-4-methyloctanoic acid and its ethyl ester is presanted starting from n-hexanal. The key stefJ in this sequence is an orthoester Clalsen rearrangement for the elongation of the carbon chain by two.