The catfish Pangasius hypophthalmus from South-Eastern Asia uses the airbladder as an accessory respiratory organ. The airbladder is situated at the dorsal part of the body cavity and is connected with the esophagus by a short pneumatic duct lined with mucus cells. The histological and ultrastructural study reveals that the wall of the airbladder is composed of three layers: the outer layer of serosa membrane, the middle one of connective tissue with many collagen fibres, and the inner layer - the simple respiratory epithelium. The medium and inner layers form numerous septa protruding into the lumen of the airbladder. The central septum divides the airbladder longitudinally into two chambers. The respiratory epithelium consists of a single type of flat cells covering a dense capillary network. Ciliated and mucus cells are absent. Epithelial cells possess long and thin microvilli on their surface. Their cytoplasm contains a large nucleus, the Golgi apparatus, rough endoplasmic reticulum, mitochondria, and a few electron-dense and lamellar bodies accumulating surfactant. The air-blood barrier is composed of epithelium, connective tissue, and endothelium with a thickness ranging from 0.2 to 2.4 mum.