The ultrastructural lung pathology of Swiss Webster mice on days 2, 4, 7, 11, 15, and 21 after intranasal inoculation of Chlamydia pneumoniae AR-39 is described. The inflammatory infiltrate was predominantly polymorphonuclear leukocytes on day 2. By day 7, mononuclear cells were most prominent in the infiltrate. On day 2, chlamydial inclusions were found frequently in the bronchial ciliated epithelial cells and less frequently in the interstitial cells that appeared to be macrophages. Free particles of all developmental forms of the chlamydial microorganism were found in the bronchial lumen and alveolar space. These particles were likely organisms released from infected cells. Inclusions as well as free particles were difficult to find after day 4. These ultrastructural observations suggest an immunopathologic basis for the acute phase of the disease process.