The present paper deals with the histological structure of the swimbladder and uhramicroscopic structure of mucosa (also called respiratory-epithelium) of an air-breathing catfish,Pangasius sutchi, of which the swimbladder as an obligatory respiratory organ. The structure of pneumatic duct of the fish is similar to that of the esophagus, being organized by mucosa, submucoa, muscularis and fibrosa. Formed mainly by goblet and epithelial cells, the mucosa is stratified, with longitudinal folds. Muscularis consists of well-developed circular skeletal muscle, especially in posterior part of the pneumatic duct, where the muscle is so welldeveloped that it forms a larynx-like structure to regulate the amount of air inlet and outlet of the swimbladder. The trabeculated swimbladder is composed of three parts; mucosa, collagenic fiber wall and fibrosa. The wall of swimbladder and the respiratory alveoli consist of collagenic fibers but no muscle fibers are present. The mucosa is a sort of highly vascularized respiratory epithelium with capillaries covering the luminal surface of the respiratory alveoli. Between the respiratory epithelium and collagenic fibers, there is a relatively thin layer of loose connective tissue within which fibrohlasts, various kinds of white blood corpusles and blood vessels Were found.The respiratory epithelium is a very thin cellular structure, with a layer of simple squamous epithelial cells and the endothelial cells closely connected to the basement of the epithelial cell. A rather thin basement membrane separates the epithelial cells from the endothelial cells. Hence, the blood-gas barrier for aerial exchange is composed of epithelium, basement membrane and endothelium. All the squamous epithelial cells are isotypic having T-shaped nuclei surrounded by endoplasmic reticulum and osmiophil multilamellar bodies. Mitochondria and Golgi apparatus are seen, often with vacuoles of different sizes. From the apices of the epithelium, short microvilli extend irregularly into the alveoli space. The characteristics of the epithelial cell mentioned above are similar to type Ⅰ and type Ⅱ cells of the alveolar epithelium in mammals. In this paper, the ultramicroscopic structure of endothelial cells, mast cells, and fibroblasts are also described, and the relations of the swimbladder to aerial respiration are discussed.