We report the first record of blow flies (Protocalliphora) parasitizing Swainson's Warblers (Limnothlypis swainsonii). Eight of 12 (67%) nests collected in southeastern Oklahoma during four breeding seasons (2001–2004) were parasitized by P. deceptor larvae. Because Swainson's Warbler is considered a species of high conservation priority in the southeastern United States, and because Protocalliphora can have negative impacts on their hosts, factors influencing blow fly parasitism of this species warrant further investigation.