Papilio memnon is a swallowtail butterfly widely distributed in south-east Asia. The females are highly polymorphic and many of them are mimetic. The mode of inheritance of seventeen of the female forms is reported. In contradistinction to earlier work it has been shown that they are controlled by what appears to be a series of at least eleven autosomal alleles at one locus, sex-controlled to the female in effect. There is evidence, however, that the locus is complex, comprising at least three closely linked loci with occasional occurrence of crossing over between them. Two characters which are not polymorphic and one which may be polymorphic are controlled by genes unlinked to the complex locus (the super-gene). In general, dominance is complete between sympatric forms but absent when they are allopatric. The resemblance between the mimetic forms of P. memnon and their models is greater in the gene-complex of a race in which the allelomorph occurs than in hybrids with a race in which it does not. Thus in no case is the resemblance better in the race cross, in ten cases there is no change and in thirty-five the mimicry is less good. The genetic control of the polymorphism in P. memnon shows remarkable parallels with that in P. dardanus and provides further supporting evidence for Fisher's and Ford's view that mimicry evolved gradually by adjustment of the gene-complex as a result of natural selection favouring those wing patterns which most closely resembled the models. Furthermore, as in P. dardanus, the mimicry is controlled by what appears to be a super-gene, adding weight to the conclusion that the genetic control of the polymorphic Batesian mimicry has evolved gradually by the accumulation of closely linked allelomorphs in advantageous combinations. This contrasts with the genetic control of Mullerian mimicry as evidenced in the Heliconids. In P. memnon the dominance relationships of the monomorphic tailed and tailless condition (excluding the form achates) indicate that dominance can be evolved even when the characters concerned are not polymorphic. In addition, the lower frequency of dominance between allopatric forms than between sympatric ones is strongly in favour of the view that dominance has evolved. Similar evidence has been found from breeding work in the Heliconids and in P. dardanus; however, the phenomenon is not confined to mimetic situations since there is also evidence for the evolution of dominance in other polymorphisms including industrial melanism.