Leech species of the mid-Snake River of Idaho and Oregon are described, and the distribution of the extant leech Mooreobdella microstoma Moore in the Snake River paleodrainage is delineated. Samples were collected from aquatic surveys in the Snake River using suction dredging by the Idaho Power Company and U.S. Bureau of Reclamation between 1995 and 2006. Supplementing these surveys, opportunities were provided for leech identification in water-quality analyses in Arizona and Wyoming and in other surveys in California, Oregon, Washington, and Idaho. Eight species of leeches were found in the Snake River surveys. Erpobdella parva Moore was the most widely distributed species, occurring both above and below Shoshone Falls. Mooreobdella microstoma was widely distributed below Shoshone Falls. Other leech species were rare, although Helobdella stagnalis Linnaeus was very common above and less common below Shoshone Falls, a natural barrier to the anadromous fish. Mooreobdella microstoma is an extant species that links the Snake River to the lower Colorado River by various paleodrainages. It probably colonized the Snake River by upstream movement, whereas Erpobdella parva likely colonized this river by downstream movement.