The air chambers of the newborn opossum are lined by a respiratory epithelium which appears similar to that lining alveoli of other species. The type II pulmonary epithelial cell is cuboidal in shape, shows apical microvilli, several Golgi complexes, lipid droplets, and numerous cytosomes. The cytosomes contain osmiophilic lamellae and are similar in appearance to whorls of osmiophilic material lying free within the alveolar lumina and on the surface of the respiratory lining epithelium. In the newborn opossum lung, examination of the air chambers with the scanning electron microscope permits the observation of several type II pulmonary epithelial cells in a single field. The potential use of the air chamber of the newborn opossum as an experimental model is suggested.