Cytoplasmically interited microorganisms are widespread in insects and have been implicated as causes of female parthenogenesis (females developing from unfertilized eggs) and cytoplasmic incompatibility. Normal sexual reproduction can be restored by treatment with antibiotics. Sequence analysis of the DNA encoding 16S ribosomal RNA has shown that cytoplasmic incompatibility bacteria from diverse insect taxa are closely related (they share >95% sequence sililarity) and belong to the alpha subdivision of Proteobacteria. Here we show that parthenogenesis-associated bacteria from parasitoid Hymenoptera also fall into this bacterial group, having up to 99% sequence similarity to some incompatibility microorganisms. Both incompatibility and parthenogenesis microorganisms alter host chromosome behaviour during early mitotic divisions of the egg. Incompatibility bacteria act by interfering with paternal chromosome incorporation in fertilized eggs, whereas parthenogenesis bacteria prevent segregation of chromosomes in unfertilized eggs. These traits are adaptive for the microorganisms. On the basis of their sequence similarities, we conclude that parthenogenesis bacteria and cytoplasmic incompatibility bacteria form a monophyletic group of microorganisms that 'specialize' in manipulating chromosome behaviour and reproduction of insects.