The lobular division, bronchial tree, and blood vessels in lungs of seven squirrel monkeys (Saimiri sciureus) were examined from the viewpoint of comparative anatomy. The right lung of the squirrel monkey consists of the upper, middle, lower, and accessory lobes, whereas the left lung consists of the upper, middle, and lower lobes. These lobes are completely separated by interlobular fissures. In three of seven examples examined the left middle lobe was lacking. The squirrel monkey lung has four bronchiole systems, i.e. dorsal, lateral, ventral, and medial, on both sides. The upper lobes are formed by the first branches of the dorsal bronchiole systems. The middle lobes are formed by the first branches of the lateral bronchiole systems. The remaining bronchioles constitute the lower lobes. In addition to the above lobes, in the right lung, the accessory lobe is present, being formed by the first branch of the ventral bronchiole system. The right pulmonary artery runs across the ventral side of the right upper lobe bronchiole, and then across the dorsal side of the right middle lobe bronchiole. Thereafter, it runs between the dorsal bronchiole and lateral bronchiole systems along the dorso-lateral side of the right bronchus. During its course, the right pulmonary artery gives off the arterial branches which run along each bronchiole. These branches run mainly along the dorsal or lateral side of the bronchioles. In the left lung, the pulmonary artery and its branches run the same course as in the right lung. The pulmonary veins run mainly the ventral or medial side of the bronchioles, and between the bronchioles.