Abstract: Morphological and functional aspects of the urinary system of Monopterus albus (Zuiew) are dealt with. The hollow, tubular, blind-ending structure situated on the left side of the abdominal cavity of M. albus is not a degenerate gonad, but a specialized urinary bladder. The inner surface of the bladder has plenty of villi with transitional epithelium. Due to the richness of capillaries either in the villi or at the base of the structure, the urinary bladder is considered to have its function not only for storage, but also as an organ of reabsorption. At the connection between mesonephric canal and urinary bladder, the lumen of the bladder has a distinct longitudinal dorsal fold, which appears to be a huge villus in cross-section of the bladder. The forming of such huge villus is mainly due to the insertion of the mesonephric canal.The kidney of M. albus is long and thin, Y-shaped, and adhering to the dorsal surface of the abdominal cavity. The anterior part is the head kidney, having no nephron but hemopoietic tissue. The posterior part is the trunk kidney, possessing nephrons similar to that of mammals but fewer in number. The renal tubule consists of neck segment, primary proximal segment, secondary proximal segment, primary distal segment and secondary distal segment. Two mesonephric canals lie in the ventral inner border of the trunk kidney, with epithelium either transitional or pseudostratified ciliated columnar.The tissue interlocated between renal tubules of the fish was found containing numerous immature red blood cells, while the head kidney seems to be the tissue for storing or releasing the mature red cells. Therefore, the kidney may be the main hemopoietic organ of M. albus.