A mouse model for studying infections due to Chlamydia trachomatis is described. Pneumonitis was produced by intranasal inoculation of four trachoma and one lymphogranuloma venereum strains. The infection was confirmed by cell culture isolation of the organisms from the lung, detection of serum antibody and delayed hypersensitivity, and the observation of inclusions in the interstitial cells of the lung by light and electron microscopy. This study indicates that mice may serve as a useful nonprimate animal model for the study of the pathogenesis and immunology of C. trachomatis infection.