Inherited bacteria which kill males during early development are widely dis
tributed throughout the insects, but have been little studied outside of a
single family of beetles, the Coccinellidae. We have investigated a male-ki
lling 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 com
mon ancestor of the butterflies. The prevalence of Wolbachia is remarkably
high, 95% of females are infected and only 6% of wild caught butterflies ar
e male. Measurements of the vertical transmission efficiency were used to c
alculate that this high prevalence is the result of infected females produc
ing at least 1.79 times the number of surviving daughters as uninfected fem
ales (lower confidence limit is 1.25).