Dactylogyrus extensus is a monogenean trematode gill parasite of Common Carp (Cyprinus carpio). It was described from Lake Oneida, New York in 1932, but was clearly introduced to North America, with its host, in the 19th century. In North America, it has been collected from carp in Lakes Huron, Erie, and Ontario, from the Missouri River in North Dakota, the Little Colorado River in Arizona (Choudhury et al. 2004). It is now widespread across Europe in wild and cultured carp, and has become established in Australia. It has been reported in a survey of fish parasites at the Tracy Pumping Plant, in the Sacramento-San Joaquin Delta in 1972-73, in the gills of Common Carp.