The pupae of the tropical butterflyDanaus chrysippusare either green or pink the switch being operated by a ‘greening’ hormone produced in the larval head. Both environmental and genetic cues are involved in controlling the endocrine mechanism.The environmental factors identified are of two distinct kinds: proximate factors influence pupal colour after the larva has selected its pupation site, whereas ultimate factors are effective at an earlier stage, either prompting choice of pupation site by the larva or priming pupation physiology in a particular direction. Genetic factors preadapt the larva to form a pupa which will be cryptic in the normal or average conditions, climatic or biogeographical, anticipated in its environment.The proximate factors demonstrated are background colour, darkness, light quality (wavelength) and humidity. There is some evidence that substrate texture may also be relevant. Ultimate factors are temperature, humidity and species of larval foodplant. Two closely linked gene loci which govern the phenotype of adult morphs and races either have a pleiotropic effect on pupa colour or are closely linked with other genes which do so. Moreover, the two loci interact epistatically with respect to their pupation effects.Factors producing predominantly green pupae are plant substrates, yellow background, darkness, yellow light, high humidity, high temperature, theballele at the B locus when homozygous and, on non‐plant substrates, theCallele at the C locus. High frequencies of pink pupae result on non‐plant substrates, red backgrounds, in blue light, low humidity, low temperatures and inB‐andccgenotypes. The C locus alleles,Candc,interact epistatically with the B alleles, their effect on choice of pupation site being determined by linkage phase. Of the two foodplants tested,Calotropisproduced a high frequency of green pupae andTylophoraof pinks. The seasonal cycling of rainfall, temperature, availability or condition of foodplant, and gene frequencies are all correlated with oscillations in the frequencies of green and pink pupae. Though genotype influences pupa colour, all genotypes are capable of forming pupae of both colours. The variation can therefore be attributed to an environmental polyphenism superimposed upon a genetic polymorphism.The hormone producing green pupae emanates from the head during the prepupal period. Denied hormonal influence, the pupa is pink. Pupal colour is judged to be aposematic at close range and cryptic at