Outbreaks of a disease characterized by severe necrotic enteritis occurred among Canada geese (Branta canadensis), lesser snow geese (Anser caerulescens caerulescens), Ross geese (A. rossi), and white-fronted geese (A. albifrons) on lakes in Saskatchewan and Manitoba during the autumn of 1983, 1984 and 1985. Ducks using the lakes were apparently not affected. Lesions in the geese closely resembled those described in enteritides in other species associated with the proliferation of Clostridium perfringens in the small intestine. Clostridium perfringens was present in large numbers in the affected areas of the intestine of the geese; other pathogens were not identified. It is hypothesized that an abrupt change in diet as geese begin to feed on grain disrupts the intestinal microflora, allowing C. perfringens to proliferate in the upper small intestine. Toxins produced by the bacteria then cause mucosal necrosis. Protease-inhibitory substances in some grains might also have a role in the disease.