Heavy losses of largemouth bass fingerlings at the Jonkershoek Inland Fish Hatchery, Stellenbosch, South Africa, were attributed to heavy infestation of the ureters with a gyrodactyloid parasite closely related to Acolpenteron ureterocoetes. The parasite has no intermediate host, and in heavily infested fish may complete its entire life cycle in the ureters. Infected fish display inflammation of the kidneys, ureters, and urinary openings and, in acute cases, great abdominal distension anterior to the cloaca. Some diseased fish die gradually, others in convulsive spasms. This disease occurs only in fish reared on artificial food in concrete rearing tanks. The spread of the disease can be checked by transferring fish from such small tanks to fertilized open ponds, where natural food and absence of crowding usually alleviate the condition. It is recommended that largemouth bass be reared to the fingerling stage in fertilized earthen ponds 0.1 acre or more in size, stocked at the rate of 12,000 advanced fry per acre and drained after about two months, when a survival of 80 percent or more may be expected.