Clostridium piliforme infection (Tyzzer's disease) was diagnosed in a colony of nude mice. Because spontaneous Tyzzer's disease had not been reported in nude mice, a study was undertaken to better define the clinicopathologic features of this disease outbreak. Sixty homozygous nude (nu/nu) females, 10 nu/nu males, and 10 heterozygous nude (nu/+) females were observed for signs of disease. Over a 3-month period, 43% of the nu/nu mice died or manifested clinical signs of disease and were euthanized, but nu/+ mice remained healthy. Clinical signs of disease were infrequently observed in nu/nu mice and, when evident, were followed by rapid deterioration and death. Gross and histologic lesions, including severe hepatic and intestinal necrosis associated with C. piliforme, were observed only in clinically affected animals. Clostridium piliforme isolated from diseased livers had marked cytotoxicity in in vitro assays. This outbreak is unique in that, contrary to a previous experimental report, nu/nu mice had increased susceptibility to Tyzzer's disease, suggesting that T cells may play an important role in host defenses against C. piliforme infection. In addition, this is the first report of a toxigenic isolate of C. piliforme recovered from mice. The cytotoxin produced by the isolate may have contributed to the severity of clinical disease and lesions.