Polyphenisms are evolved adaptations in which a genome produces discrete alternative phenotypes in different environments. In this study, the genetic basis of the evolution of a polyphenism by genetic accommodation was investigated. A polyphenic strain and a monophenic strain of Manduca sexta (L.) were crossed and the F(1) offspring and backcross progeny were analysed. The larval colour polyphenism was found to be regulated by one sex-linked gene of major effect and many smaller effect modifier genes. The finding shows that the mechanism of genetic accommodation relies on genetic changes that are consistent with the current view of the genetic basis of adaptive evolution.