Under natural conditions, eukaryote cells may contain bacteria. Arthropods such as ticks, insects or mites are a group particularly favoured by the obligate intracellular bacteria. While arthropods are vectors for some of them, other bacteria inhabit invertebrate host cells having entered mutualistic interactions. Such endosymbionts dwell usually in the host cell vacuoles or cytoplasm but have been also reported from tick oocyte mitochondria. The microorganisms contribute to mitochondria degradation, but their colonies are not eliminated from the tick cells affected. So far, such bacteria have been detected in three research centres. The Italian centre has reported on results of microscope and molecular analyses, while the Polish centre published molecular data. The Danish centre registered a 16S rRNA gene fragment in GenBank. Independent comparisons of the 16S rRNA gene sequences, carried out in the Italian and Polish centres, confirmed that the nucleotide sequences of the Ixodes ricinus endosymbionts formed a single clade with certain non-identified tick bacterium species isolated from the tick Haemaphysalis wellingtoni. On the other hand, pathogenic species of the genera Anaplasma, Ehrlichia and Rickettsia detected in I. ricinus as well as symbionts of the genus Wolbachia present in Culex pipiens and Drosophila simulans have been placed at a different site on the phylogenetic tree.