In eastern North America and Cuba, King and Clapper Rails (Rallus elegans and R. longirostris) are fresh- and saltwater counterparts that differ in plumage, size, development of the salt gland and width of the interorbital bridge, and in certain aspects of behavior. King Rails are capable of inhabiting saltmarshes but seldom do so when Clapper Rails are present and may be excluded from eastern saltmarshes by Clapper Rails. On the other hand, Clapper Rails are not known to invade freshwater marshes. The two species hybridize readily, but only in areas where ecological conditions are intermediate, a specific instance of which in Florida is described in detail. Hybrids do not occur outside these areas of intermediate salinity and nowhere is there a hybrid zone in which no pure parental types occur. King and Clapper Rails are therefore best treated as separate species. Morphology, distribution, and paleontology are used to reconstruct a hypothetical evolutionary history of the complex. Rallus longirostris falls into two subspecies-groups—a heavy-billed longirostris group in South America and a slender-billed crepitans group in the West Indies and coastal marshes of eastern North America and Yucatán. All other North American populations of the complex on the Pacific coast of California and Baja California, and in central and western Mexico, are regarded as relicts of an original widespread North American King Rail stock that were isolated following desertification of the American west and should be treated as subspecies of Rallus elegans.