In my PhD thesis I have studied different aspects of the biology of the male-killing (MK) Wolbachia strain wBol1 that infects the tropical butterfly Hypolimnas bolina. First I have investigated the causes and consequences of the recent spread of wBol1 in the South Pacific Islands, bringing new insights into the biogeography of this bacterium. Theoretical studies predicted that MK symbionts could not spread to extremely high frequency in their host populations without driving the host species to extinction due to a severe shortage of males; I suggested however that wBol1 may transgress this rule by slightly enhancing its host‟s fitness and making the infected butterflies more competitive than their uninfected counterparts. I therefore explored the effects of different Wolbachia infections in hosts affected by parasitoids and demonstrated that Wolbachia decreases parasitoid success in flies, and suggested that similar protection may occur in wBol1-infected butterflies. At the start of my PhD, no genomic data was available for MK Wolbachia. By sequencing the wBol1 genome, I have produced the first MK Wolbachia genome to date. By comparing the genome of wBol1 to that of its close relative, wPip, the symbiont of Culex pipiens, I identified genomic features common to both B-group Wolbachia, as well as genes unique to this MK strain. The wBol1 genome contains, for example, three prophage regions, the largest number of ankyrin coding genes so far recorded for a Wolbachia strain and a large number of transposases potentially involved in genome rearrangement events. This first analysis of the wBol1 whole genome sequence is an important resource for future ecological, evolutionary and mechanistic studies examining MK Wolbachia biology.