AbstractCytoplasmic incompatibility results in embryo mortality in diploids, or all male offspring in haplodiploids, when individuals carrying different cytoplasmic factors are crossed. Cytoplasmic factors have been identified as intracellular micro‐organisms. Microbeinduced cytoplasmic incompatibility is found in many insect taxa and may play a role in reproductive isolation between populations. Such micro‐organisms cause bidirectional incompatibility between species of the parasitoid wasp genusNasonia. The phylogenetic relationship of cytoplasmic incompatibility microorganisms (CIM) of differentNasoniaspecies was analysed using their 16S ribosomal DNA (rDNA) sequence. Two 16S rDNA operons were detected in the CIM of eachNasoniaspecies. Sequence analysis indicates that theNasoniaCIM are closely related and belong to the alpha group of the Proteobacte