Th e Carabid beetle Poecilus lepidus occurs in diff erent colour forms, which are controlled by a gene with at least fi ve alleles. Spectra of the colour forms were measured and the cuticle studied by electron microscopy.Th e three colour forms (blue, green, and black) from Italy show simple Mendelian heredity with the genotypes bb, bs, and ss. Th ey display refl ection spectra, which are caused by a multilayer system of electron dense and less dense layers. Th e same system exists in German populations, but the colours are closer together (bluish green, yellowish green and red; genotypes gg, rg, and rr). In addition to this system, in German populations a violet form was found in the genetic analysis (Vr, Vg), the colour of which is also produced by a multilayer system. Th e colour measurements confi rmed that a multilayer system exists not only in the heterozygous males and females, but also in the black homozygous VV females with a maximum in the near ultraviolet. At the other end of the spectrum, at least two black forms were detected by their refl ectance peaks in the infrared range. Th ese peaks are beyond the visible spectrum and are also due to a multilayer system. However, the heredity of these forms is unknown.