Melanism in Lepidoptera, either industrial or in mimicry, is one of the most commonly cited examples of natural selection [1] [2]. Despite extensive studies of the frequency and maintenance of melanic genes in insect populations [1] [2], there has been little work on the underlying molecular mechanisms. Nowhere is butterfly melanism more striking than in the Eastern Tiger Swallowtail (Papilio glaucus) of North America [3] [4] [5]. In this species, females can be either yellow (wild type) or black (melanic). The melanic form is a Batesian mimic of the distasteful Pipevine Swallowtail (Battus philenor), which is also black in overall color. Melanism in P. glaucus is controlled by a single Y-linked (female) black gene [6]. Melanic females, therefore, always have melanic daughters. Black melanin replaces the background yellow in melanic females. Here, we show that the key enzyme involved is N-beta-alanyl-dopamine-synthase (BAS), which shunts dopamine from the melanin pathway into the production of the yellow color pigment papiliochrome and also provides products for cuticle sclerotization. In melanic females, this enzyme is suppressed, leading to abnormal melanization of a formerly yellow area, and wing scale maturation is also delayed in the same area. This raises the possibility that either reduced BAS activity itself is preventing scale sclerotization (maturation) or, in contrast, that the delay in scale maturation precludes expression of BAS at the correct stage. Together, these data show how changes in expression of a single gene product could result in multiple wing color phenotypes. The implications for the genetic control of mimicry in other Lepidoptera are discussed.