An isolate of Chlamydia trachomatis obtained from a man with nongonococcal urethritis was used to produce experimental ure-thral and pharyngeal infections in chimpanzees. After urethral inoculation of only 8 χ 101 inclusion-forming units (IFU), infections were established in three of three animals; urethral discharges developed in two. The infections persisted for five to nine weeks. Larger inocula (7 χ 102 and 1 χ 105 IFU) produced pharyngeal infections in two animals. The third animal's pharynx was not infected by 1 χ 105 IFU. Chlamydial complement-fixing antibodies increased significantly in sera of two of three animals. This study provides an animal model for study of mucosal infection by C. trachomatis. The relative resistance of the chimpanzee pharynx to infection parallels clinical observations in man.