As mitochondria are inherited in a matrilinear way, an animal hybrid contains the mitochondrial DNA of its 'mother species'. Of 80 studies that analysed the mitochondrial DNA of at least five hybrid individuals, 50 showed that all hybrids contained the mitochondrial DNA of only one of the two parental species, indicating either mating of females of species A with males of species B but not vice versa (unidirectional hybridization) or the disappearance of one of the two parental mtDNA types after reciprocal hybridization. I review and discuss factors promoting unidirectional or reciprocal hybridization and present a sexual selection hypothesis for unidirectional hybridization. The inequality of the sexes in parental investment leads to the sex that invests more being the more discriminating one. In the presence of conspecific males, females reject allospecific males and, consequently, a male in an environment of both allospecific sexes is unlikely to mate, while in the absence of conspecific males, females sometimes accept fertilizations by males of other species. Thus, hybrid matings are usually between the females of a rare species and the males of a common species, but not vice versa. Copyright 1999 The Association for the Study of Animal Behaviour.