Two commercial White Leghorn flocks of 41,000 and 57,000 25-week-old hens experienced death losses of 676 and 1,089 birds in a week. Six birds from each flock were presented for necropsy to the Animal Disease Diagnostic Laboratory at Purdue University with a clinical history of excessive deaths, unsatisfactory production, and dehydration. Gross lesions were present on the tongue and on oral and pharyngeal mucosa as uniformly pale or white areas with interspersed multifocal sites of ulceration. Other changes were cyanotic, shriveled combs and wattles and ruptured ovarian follicles with concurrent egg yolk peritonitis. Microscopic examination revealed large areas of coagulative necrosis with multifocal epithelial ulceration of the oral and pharyngeal mucosae. There was an intense inflammatory, heterophilic response at the periphery of affected areas. Alterations in the abdominal viscera were limited to egg yolk peritonitis. The high death rate and oral lesions were attributed to accidental use of excessive levels of a quaternary ammonium disinfectant in the drinking water.