A diagnosis of Tyzzer's disease (Bacillus piliformis infection) was made in a group of recently-trapped muskrats (Ondatra zibethica). The major lesions consisted of enteritis and focal necrosis of the liver, with organisms resembling B. piliformis in hepatocytes on the periphery of these foci. The outbreak was associated with recent capture, housing, and the use of sulfaquinoxaline medicated feed.