Bacterial endosymbionts with significant homology to Francisella tularensis (γ-proteobacteria) have been described from at least five species of ticks in three different genera, including two North American Dermacentor species [D. andersoni Stiles and D. variabilis (Say)]. The evolutionary relationships among the Francisella-like endosymbionts (FLE) from different hosts and between FLE and the arthropod-borne pathogen F. tularensis are not known. A 1,169-base fragment of the 16s rDNA and a 713-base fragment of the F. tularensis 17-kDa lipoprotein gene homolog of the FLE of six North American Dermacentor tick species [D. anderson, D. variabilis, D. albipictus (Packard), D. occidentalis Marx, D. hunteri Bishopp, and D. (Anocentor) nitens Neumann] and of Amblyomma maculatum Koch and Ornithodoros porcinus (Murry 1877, sensu Walton 1979) as outgroups, were subjected to phylogenetic analysis. These gene phylogenies were compared with a phylogeny of the same tick species constructed from a 435-base fragment of the tick mitochondrial 16s rDNA. Although the phylogenies of the FLE and their tick hosts are parallel at the genus level, the Dermacentor FLE are unresolved at the species level. The FLE and the Dermacentor ticks show little sign of co-speciation, possibly indicating that the association between these endosymbiont and the Dermacentor ticks is of a relatively recent origin. Several ticks were co-infected, either with two FLE with divergent 17-kDa lipoprotein gene sequences or with FLE and an unidentified species of spotted fever group rickettsia (α-proteobacteria). Infection with FLE does not seem to have precluded infection with either a second closely related γ-proteobacterial symbiont or with a second less closely related α-proteobacterial symbiont.