Females of the butterfly Acraea encedon produce either entirely female offspring or males and females in an almost 1:1 sex ratio. The sex ratio produced is maternally inherited and was previously attributed to sex chromosome meiotic drive. We report that all-female lineages are associated with low egg-hatching rates and that the trait is cured by antibiotic treatment. We thus reject the hypothesis that this sex ratio bias is caused by a meiotically driven sex chromosome and, instead, propose that it is associated with a maternally inherited bacterium that kills males.Keywords: Acraea encedon, male killing, meiotic drive, sex ratio