The genetics of eight sympatric colour forms of the polymorphic butterfly Danaus chrysippus are investigated. Colour is controlled at three loci. The B locus controls the ground colour of both wings, which may be either orange or brown, the latter being dominant. The C locus controls the forewing pattern characterized by either a black apical patch with a row of white spots (form chrysippus) or orange or brown colouring throughout (form dorippus). The latter is dominant. At the A locus, the presence of a large white patch on the hindwing (giving form alcippus when combined with a chrysippus-type forewing or form albinus with a dorippus-type forewing) is recessive to its absence. Dominance is often incomplete at both the A and C loci so that many heterozygotes are phenotypically distinct. The B and C loci are closely linked and probably segregate independently from the A locus. There is evidence suggesting that form chrysippus has superior viability to form dorippus in reared broods. At the A locus, heterosis may be involved. The mode of origin and maintenance of the polymorphism is discussed.