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The systematic relationship between the Gulf saltmarsh snake, Nerodia fasciata clarkii. and the freshwater subspecies, N. f. confluens. N. f. fasciata, and N. f. pictiventris, is re—evaluated using morphological characters that reflect head shape. Changes in character-states are analyzed from areas of contact into the populations of the freshwater forms. Beyond the narrow zones where intermediates are found there is no morphological evidence suggesting genetic introgression of clarkii genes into the freshwater populations. Discriminant analysis of the head scale measurements also suggests the significant division of the saltmarsh and freshwater forms. Ecological analysis reveals that interbreeding between the forms is usually restricted to disturbed areas where the salinity gradients are disrupted. These morphological and ecological data, in addition to the biochemical and physiological findings of other researchers, support the removal of the saltmarsh form from the status of a subspecies of Nerodia fasciata and its recognition as a distinct species, Nerodia clarkii.