The existence of a symbiotic relationship between Trichomonas vaginalis and Mycoplasma hominis, which is the first reported example of symbiosis between two obligate human pathogens, has been recently reported by our research group. In this work, we examined the cellular location of M. hominis in respect to T. vaginalis. By using gentamicin protection assays, double immunofluorescence, and confocal microscopy, we obtained strong evidence that M. hominis is located within protozoan cells. 5-Bromodeoxyuridine incorporation assays showed that intracellularly located mycoplasmas actively synthesize DNA. Our results demonstrate that M. hominis has the capability of entering trichomonad cells and of replicating inside the protozoon. These findings suggest that symbiosis might provide the bacteria, during human infection, with the capability to resist to environmental stresses, such as host defense mechanisms and pharmacological therapies.Trichomonas vaginalis is a parasitic protozoon responsible for trichomoniasis, one of the most common sexually transmitted diseases in humans, estimated to affect at least 200 million people worldwide (37). Studies carried out with large groups of women suffering from vaginitis showed that T. vaginalis is clinically associated with Mycoplasma hominis (18, 35), a bacterium that, like the protozoon, resides exclusively in the human genital tract. The association is strictly species specific, since is not observed with Ureaplasma urealyticum, another Mollicutes species that is a much more common inhabitant of the human genital tract. The clinical association between the two pathogens has been recently explained by the demonstration of a symbiotic relationship between T. vaginalis and M. hominis (27). More than 90% of T. vaginalis clinical isolates from our collection proved to be infected by M. hominis independent of their geographic origin. Our recent work has allowed us to shed light on some aspects of the phenomenon (28).The presence of endosymbionts in free-living protozoa is frequently described, but it has been never reported in obligate parasitic protozoa. The first example of symbiosis involving a human pathogen was described for Legionella pneumophila, a bacterial pathogen that is responsible for Legionnaire's disease, and Acanthamoeba sp., a free-living opportunistic amoeba (30). The relationship between T. vaginalis and M. hominis is the only one described so far involving two obligate human pathogens. T. vaginalis is responsible for severe vaginitis accompanied by abdominal pain, itching, and foul-smelling discharge (29) and is mainly asymptomatic in men (20). Moreover, trichomoniasis is associated with an enhanced risk of neoplastic transformation in cervical tissues (38) and increased human immunodeficiency virus seroconversion in women (22,31). The mechanisms by which T. vaginalis exerts its pathogenic effects involve adhesion to host cells (1, 2, 19) and the activity of pH-dependent pore-forming proteins (14, 15) and of cytoskeleton-disrupting proteases (16). M. hominis c...