The ultrastructure and colour of the wing scales of Papilio zalmoxis were studied by means of scanning electron microscopy and spectrophotometry. The colour of the blue scales was found to be mainly of structural origin, due in part to Tyndall scattering by a layer of air-filled alveoli and in part to thin film interference in a basement lamella. A white, fluorescent pigment, probably composed mainly of kynurenine, lines the alveoli, and contributes to the colour principally by virtue of its absorption of ultraviolet radiation. Males and females possess these blue scales, but the overall colour they produce in the wing depends on the distribution of underlying black scales, and this differs between the sexes. The female of this species is figured in colour for the first time. The apparent polymorphism of preserved males was examined and it is concluded, with some reservations regarding the brown form, to be due to post-mortem colour changes. The nature of the coloration and the sexual dimorphism of P. antimachus are described. The sexual dimorphism of both species is discussed.