Sex ratio distortion in the tropical butterfly Acraea encedon is caused by infection with a male-killing bacterium of the genus Wolbachia. Severely female-biased populations with extremely high bacterial prevalences were previously reported in A. encedon. In this paper, we describe a study in which the spatial variations and the temporal changes in the population sex ratio and the prevalence of the male-killing Wolbachia were thoroughly investigated in the wild populations of Acraea encedon in Uganda between 2005 and 2007. The two parameters were found to vary extensively over space and time, indicating that the male-killer is highly dynamic. Both the female-bias and the Wolbachia prevalence were lower than that recorded in the literature.