Megathylacoides giganteum is a tapeworm, endoparasitic on freshwater fishes, particularly catfishes of the family Ictaluridae. The most frequent hosts are Channel Catfish (Ictalurus punctatus) and Flathead Catfish (Pylodictus olivaris). but it is also known from White Catfish (Ameirus catus), Black Bullheads (A. melas) and other species. It was described from the Rock River, Rockford, Illinois, and has been collected in at least three of the Great Lakes, in the Mississippi Basin in Wisconsin, Illinois, Iowa, Kentucky, and Tennessee, and also in northern Mexico. It has been introduced to the Colorado River basin with Channel Catfish. In 1967, it was collected at the Tracy Pumping Plant in the Sacramento-San Joaquin Delta from the intestine of White Catfish, introduced to San Francisco Bay system in the 19th century.