The aposematic savanna butterfly Danaus chtysippw seems to be an exception to the rule that Batesian models and Miillerian mimics are not polymorphic. Throughout Asia and in much of Africa the species is in fact monomorphic and polytypic, as expected. It is, however, polymorphic for 3 4 colour genes over a large area of central and eastern Africa, where deviant sex ratios and non-Mendelian segregations also occur. AU-female broods are widespread and frequent, often outnumbering bisexual progenies and giving rise to heavily female-biased populations. Full and partial sex linkage, which is not controlled from the X or Y chromosomes, and male-biased broods also occur. Genetical analysis for the BC-autosome carrying colour genes suggests there are two, probably mitochondrial, cytotypes (microbe-induced early male death syndrome is considered unlikely) and an autosomal, incompatibility (I) gene, two alleles of which are male-specific killers. F2 and backcross matings by females heterozygous at the I-locus give progenies which are either thelygenic, all males dying at or soon after hatching, or bisexual but showing full or partial sex linkage. Male death is attributed to nuclearcytoplasmic incompatibility (NCI). Females achieve reversion from a thelygenic to a bisexual line by mating with males of compatible (maternal) cytotype. A second NCI system causes