Larval Corydalidae, known as hellgrammites, are large, long-lived (1-5 years) predators found in a variety of aquatic habitats, including large rivers, intermittent streams, and ponds. Twenty-two species of Corydalidae occur in the United States; larvae of 15 species have been associated and described. We examined the morphology and distribution of the hellgrammites of the United States and developed a species key. Important diagnostic characters include head patterns and modified spiracles on abdominal segment VIII. Among the subfamily Corydalinae (dobsonflies), two species of Corydalus are fairly widespread; C. cornutus occurs east of the Rocky Mountains and C. texanus is west of the Rockies, although some there is some overlap in their ranges. In the subfamily Chauliodinae (fishflies), Orohermes, Dysmicohermes, and Protochauliodes are restricted to the Pacific coastal region. Two species of Neohermes are found west of the Rockies and three species are found east of the Rockies. Two species each of Nigronia and Chauliodes are widespread in the eastern U.S. Chauliodes are unique in their preference for lentic habitats. Larvae of two eastern species of Neohermes and four western species of Protochauliodes remain undescribed, and likely occur in overlooked habitats such as intermittent or ephemeral streams.