A data sample of 80 males and 43 females of the giant dynastine scarab Megasoma actaeon L. were analyzed with respect to scaling of the pronotum, elytra, and legs. In males, several somatic structures scale with nonisometry, and frontal and pronotal horns scale with very steep slopes to body length, whereas head width scales with negative allometry. This could indicate allocation of progressively more resources in major males to the sexually selected structures. In females, head width and the lengths of the first and third legs also scale with negative allometry to body size, whereas the depth of the abdomen scales with positive allometry. This could indicate allocation of progressively more resources in large females to reproduction. The huge size of M. actaeon notwithstanding, it would seem that it is more primitive in terms of sexually selected combat structures than equally large species of the closely related Chalcosoma Hope and Dynastes Kirby.