Male and female animals were inoculated urogenitally with Mycoplasma genitalium, recovered originally from men with nongonococcal urethritis. Mice, hamsters and male rhesus monkeys (Macaca mulatta) were resistant. Male cynomolgus monkeys (Macaca fascicularis) were not as sensitive as male chimpanzees (Pan troglodytes): 9 of 11 developed an obvious genital tract infection, some shedding organisms for more than 18 weeks. M. genitalium was recovered from the blood of two of them when large numbers of organisms were in the urethra. Most of the chimpanzees colonized with the organisms had increased numbers of polymorphonuclear leukocytes in the genital tract and developed a fourfold or greater antibody response. Female squirrel monkeys (Saimiri sciureus) and female tamarins (Saguinus mystar) exhibited low-level genital tract infections following intravaginal inoculation, whereas marmosets (Callithrix jacchus) and chimpanzees developed prolonged infections after similar inoculation; thus, female chimpanzees shed organisms for 12 to 15 weeks. Marmosets and grivet monkeys (Cercopithecus aethiops) developed salpingitis with antibody responses after intraoviduct inoculation, and baboons (Papio anubis) developed parametritis after intracervical inoculation. The results offer substantial evidence for the pathogenicity of M. genitalium for the urogenital tract of subhuman primates, and suggest that the microorganism may have a role in human genital tract infections.