Hypolimnas bolina is a Nymphalid butterfly having a west to east distribution from Madagascar to Easter Island, and a north to south one from Japan to Australasia. It is highly migratory in some areas. In much of the western part of its range the female is both monomorphic and a mimic of Euploea. Further east it is frequently polymorphic with the majority of the forms being non-mimetic. The polymorphism is sex-limited to the female and controlled by two unlinked loci, one with two allelomorphs, E and e, determining the extent of the dark pigmentation, the other with three allelomorphs, P, P<latex>$^n$</latex> and p, determining the presence and distribution of orange-brown. Only butterflies of the genotypes EEpp and to a lesser extent Eepp are satisfactory Batesian mimics of their Euploea models. The details of the mimetic pattern are under multifactorial control, following those of their local model, as is much of the variation within the non-mimetic forms, particularly with regard to the distribution of white and blue scaling. The mimetic form is dominant or semi-dominant, depending on the background genotype, and there is little epistatic interaction between the gene responsible and the allelomorphs at the second locus. Here, the form with the greatest amount of orange-brown pigment is to all intents and purposes dominant to the other, and that with restricted brown pigment is semi-dominant to the form without any, although in certain genotypes it is entirely recessive. Unlike the situation found in previous studies of Papilio dardanus, Papilio memnon and Papilio polytes, the sex-controlled polymorphism is not determined by a supergene. It is suggested that the absence of such a supergene results from the fact that there is only one mimetic form and that over much of the species' range the butterfly is monomorphic for this. Thus the polymorphism at two or more loci necessary for the selection of increased linkage is absent throughout much of the range of the species. The genetic control of the mimicry has more in common with that in Papilio polyxenes, where there is also only one model but in which there is no polymorphism throughout the whole of the species' range, rather than in only a large part of it as in H. bolina.