Three infant male baboons were inoculated with a strain of CHLAMYDIA TRACHOMATIS ISOLATED FROM A HUMAN INFANT WITH PNEUMONITIS. One baboon, inoculated by intratracheal, nasopharyngeal, and oropharyngeal seeding, had rales, radiographic evidence of pneumonia, persistent nasopharyngeal C. trachomatis infection, and a four-fold rise in titer of antibody. At sacrifice 24 days after inoculation, nasopharynx, trachea, airways, and lung yielded C. trachomatis, and epithelial inclusions were seen by light and immunofluorescent microscopy. Histopathologic changes noted were nearly identical to those in a lung biopsy specimen from a human infant and pneumonitis and nasopharyngeal C. trachomatis. The second baboon was inoculated by tracheal seeding and maintained nasopharyngeal C. trachomatis until killed 30 days later. Autopsy revealed nasopharyngitis and patchy mild pneumonitis. The third baboon was inoculated by nasopharyngeal seeding and maintained nasopharyngeal C. trachomatis for 49 days. Both of the latter baboons seroconverted. Infant baboons appear to be useful animal models for C. trachomatis nasopharyngitis and pneumonia.