In May 1985, epizootic mortality was reported in common egrets (Casmerodius albus) on Avery Island, Louisiana. Subsequent investigation revealed that more than 400 birds died. Severe peritoneal nematodiasis (Eustrongylides sp.) was found on postmortem examination. A nearby breeding rookery on the same island was apparently unaffected. Reasons for this selective mortality are presented. Three other reports of epizootic mortality due to Eustrongylides sp. have been reported. This is the first report of this type of epizootic in gulf-coast birds.