During the summer of 1976, an epornitic of verminous peritonitis caused by Eustrongylides ignotus resulted in large scale mortality of young herons and egrets on Pea Patch Island, Delaware. Mortality was highest (84%) in snowy egret nestlings (Egretta thula) and less severe in great egrets (Casmerodius albus), Louisiana herons (Hydranassa tricolor), little blue herons (Florida caerulea), and black crowned night herons (Nycticorax nycticorax). Most deaths occured within the first 4 weeks after hatching. Migration of E. ignotus resulted in multiple perforations of the visceral organs, escape of intestinal contents into the body cavity and subsequent bacterial peritonitis. Killifish (Fundulus heteroclitus) served as the source of infective larvae.