Inherited bacteria that kill male hosts early in their development are know
n from five insect orders. We ask to what extent the incidence of male-kill
ers might be restricted by the rate at which new host-parasite interactions
arise, by testing whether multiple male-killers have invaded a single host
species. In Uganda, the butterflies Acraea encedon and A. encedana are bot
h infected by the same strain of male-killing Wolbachia and there was no ev
idence of variation within the population. In Tanzanian A. encedon however,
two phylogenetically distinct strains of male-killing Wolbachia were found
within the same population. If this pattern of male-killer polymorphism is
found to be general across infected species, it suggests that new male-kil
ling infections arise frequently on an evolutionary time scale. Whether thi
s polymorphism is stable, and what forces may be maintaining it, are unknow
n.