The right lung of the dog consists of the cranial, middle, caudal and accessory lobes. The left lung consists of the bilobed middle and caudal lobes. These lobes are separated by interlobular fissures on either side. The dog lung has four bronchiole systems, dorsal, lateral, ventral and medial, on either side. The right cranial lobe is formed by the first bronchiole of the dorsal bronchiole system. The right middle lobe is formed by the first bronchiole of the lateral bronchiole system, and the right accessory lobe is formed by the first bronchiole of the ventral bronchiole system. The remaining bronchioles of the dorsal, lateral and ventral bronchiole systems and all the bronchioles of the medial bronchiole system constitute the right caudal lobe. In the left lung, the left middle lobe is formed by the first bronchiole of the lateral bronchiole system. The remaining bronchioles of the lateral bronchiole system and all the bronchioles of the dorsal, ventral and medial bronchiole systems constitute the left caudal lobe. These findings are compared with those in other domestic animals and man.