Coxiella burnetii, the etiological agent of Q fever, is an obligate intracellular pathogen, whereas Legionella pneumophila, the causative agent of Legionnaires' disease, is a facultative intracellular pathogen. During infection of humans both of these pathogens multiply in alveolar macrophages inside a closed phagosome. L. pneumophila intracellular multiplication was shown to be dependent on the icm/dot system, which probably encodes a type IV-related translocation apparatus. Recently, genes homologous to all of the L. pneumophila icm/dot genes (besides icmR) were found in C. burnetii. To explore the similarities and differences between the icm/dot pathogenesis systems of these two pathogens, interspecies complementation analysis was performed. Nine C. burnetii icm homologous genes (icmT, icmS, icmQ, icmP, icmO, icmJ, icmB, icmW, and icmX) were cloned under regulation of the corresponding L. pneumophila icm genes and examined for the ability to complement L. pneumophila mutants with mutations in these genes. The C. burnetii icmS and icmW homologous genes were found to complement the corresponding L. pneumophila icm mutants to wild-type levels of intracellular growth in both HL-60-derived human macrophages and Acanthamoeba castellanii. In addition, the C. burnetii icmT homologous gene was found to completely complement an L. pneumophila insertion mutant for intracellular growth in HL-60-derived human macrophages, but it only partially complemented the same mutant for intracellular growth in A. castellanii. Moreover, as previously shown for L. pneumophila, the proteins encoded by the C. burnetii icmS and icmW homologous genes were found to interact with one another, and interspecies protein interaction was observed as well. Our results strongly indicate that the Icm/Dot pathogenesis systems of C. burnetii and L. pneumophila have common features.