Inherited bacteria which kill males during early development are widely distributed throughout the insects, but have been little studied outside of a single family of beetles, the Coccinellidae. We have investigated a male-killing bacterium discovered in the butterfly Acraea encedana. This bacterium belongs to the genus Wolbachia and is identical in wsp gene sequence to a male-killer in the closely related butterfly A. encedon, suggesting that it has either recently moved between host species or was inherited from a common ancestor of the butterflies. The prevalence of Wolbachia is remarkably high, 95% of females are infected and only 6% of wild caught butterflies are male. Measurements of the vertical transmission efficiency were used to calculate that this high prevalence is the result of infected females producing at least 1.79 times the number of surviving daughters as uninfected females (lower confidence limit is 1.25).