Non-Mendelian segregation involving three colour gene loci in the butterfly Danaus chrysippus is reported. In each of the four affected broods, two of the four phenotypes expected in the progeny were absent in both sexes. Operation of a system of balanced recessive lethals is ruled out by the low egg-adult mortality recorded. All gametes were produced in expected ratios and were viable. The results are interpreted to be consistent with an inherited prezygotic incompatibility mechanism which operates at the time of fertilization. Variable penetrance of the recessive a and c alleles in heterozygotes is significantly influenced, in the former case by sex, and in the latter by epistatic interactions with colour genes at the A and B loci. The combination of non-Mendelian segregation, variable incomplete dominance and epistatic interactions with other colour genes and sex supports the hypothesis, hitherto based largely on biogeographical evidence, that the polymorphic D. chrysippus population at Dar es Salaam, Tanzania may have a hybrid origin.