Abstract The symbiotic microorganisms of arthropod vectorsare highly significant from several points of view, partly due totheir possible roles in the transmission of pathogenic causativeagents by blood-sucking vectors. Although ticks are well stud-ied because of their significance to human health, novel mi-crobial associations remain to be described. This reviewsummarises several endosymbiotic bacterial species in hardticks from various parts of the world, including Coxiella-,Francisella-, Rickettsia- and Arsenophonus-like symbionts aswell as Candidatus Midichloria mitochondrii and Wolbachia.New methodologies for the isolation and characterization oftick-associated bacteria will, in turn, encourage new strategiesof tick control by studying their endosymbionts.