Localization of Mycoplasma pulmonis was examined in mice infected by direct contact with previously infected mice. After contact with infected animals, the organisms were shown to become detectable first in the nasal and oral cavities and trachea on the next day, and then they were recovered from the middle ear and brain after 3 and 4 days, respectively. After 7 days of contact, isolation rates retained to be 100% in the nasal, oral and tracheal cavities, while 95% in the middle ear and brain, 25% in the lung and 20% in the vagina and uterus. The number of colonies was the most numerous from the nasal, uterine and vaginal cavities, followed by the trachea, middle ear, oral cavity, brain and lung in this order, except for a few mice having pneumonic lesions giving a large number of the organisms. The isolation rates with these organs were not changed even after 6 weeks of contact and organisms were never detected from the liver, spleen, kidney and heart. Mice from a naturally infected breeding colony showed similar finding to those sacrificed after 6 weeks of experimental contact.