SUMMARYReproductive incompatibilities called cytoplasmic incompatibilities are known to affect a large number of arthropod species and are mediated by Wolbachia, a maternally transmitted microorganism. The crossing relationships between strains of potential hosts define their incompatibility types and it is generally assumed that differences between strains of Wolbachia induce different crossing types. Among all the described host species, the mosquito, ulex pipiens, displays the greatest variability of cytoplasmic incompatibility crossing types. We analysed mitochondrial and bacterial DNA variability in ulex pipiens in order to investigate some possible causes of incompatibility crossing type variability. We sequenced fragments of the fts gene, and the AjT-rich control region of the mtDNA. We also sequenced the second subunit of the mitochondrial cytochrome oxidase ( OII) gene, in ulex pipiens and a closely related species,. torrentium, in order to verify the usefulness of the AjT-rich region for the present purposes. No variability was found in the Wolbachia fts gene fragment, and very limited variation of the mitochondrial marker whatever the compatibility type or the origin of the host. A low variability was found in the AjTrich region and comparison of divergence of the AjT-rich region and OII gene between . pipiens and . torrentium did not reveal any special constraints affecting this region. In contrast to observations in other host species, variability of incompatibility crossing types is not due to multiple infections by distantly related Wolbachia strains.