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Abstract: The lack of phylogenetic congruence between endosymbionts and their hosts suggests that horizontal transmission of endosymbionts has occurred between species. However, the mechanisms of lateral transfer are largely unclear. Since successful transmission of infection most probably occurs when genetic distance between species is small, social parasitism between closely related species has been suggested to offer an important mechanism for interspecies transfer of Wolbachia. We compared the Wolbachia, Spiroplasma and Entomoplasma infections of the social parasite Formica sanguinea Latreille, 1798 and its Serviformica hosts to find out if horizontal transmission has occurred. We found that F. sanguinea and Serviformica mostly harboured infections with different Wolbachia strains and had significantly different Spiroplasma and Entomoplasma infection prevalences. Our results thus indicate that social parasitism between the slave-making ant F. sanguinea and its Serviformica hosts does not create substantial opportunities for symbiont transmission. The prevalence data of different Wolbachia strains suggest that infections in different Formica species are partly strain specific.