We describe the genetic control of body color polymorphism in the metallic stag beetle, Cyclommatus metallifer Boisduval. As with many species of sexually dimorphic stag beetles, male C. metallifer exhibit condition-dependent exaggerated growth of mandibles. Another characteristic of this species is that two body colors have been described from both wild and captive bred populations. The two body colors are a commonly observed gold and a more rare black. We performed a simple Mendelian cross of individuals from families that produced only gold offspring to individuals from families that produced only blue-black offspring. All of the F₁ hybrid progeny between these two pure gold families and pure black families exhibited the gold body color. The F₂ progeny resulting from the crossing of F₁ hybrids exhibited a 3:1 gold to black ratio. The backcross of the F₁ hybrid to the gold morph resulted in 1:0 gold to black ratio. The backcross of the F₁ hybrid to the black morph resulted in progeny with a 1:1 gold to black ratio. Our results show that the color polymorphism in the metallic stag beetle is under simple bi-allelic, autosomal control, and that the gold allele (G) is dominant to the black allele (g). The putative genotypes for the gold morph are GG and G_g and the blue-black morph is gg. Identification of this simple genetic mechanism for color polymorphism in the metallic stag beetle allows for future studies to use this trait as a marker in the investigation of evolutionary and ecologically relevant experiments such as those for male paternity, male mating success, and predation risk.