Bacillus piliformis infection (Tyzzer''s disease) occurred in 2 young guinea pigs, causing unthriftiness and diarrhea which resulted in death. There was necrosis and inflammation of the ileum, cecum and colon. Intestinal epithelial cells contained organisms resembling B. piliformis. Spirochetes were found in the cecum and colon, mainly in crypts. Acute diarrhea occurred in another guinea pig which became cachectic and was killed. Histologically, large numbers of spirochetes were present in the wall of the cecum and colon, and they were associated with severe necrosis and inflammation. B. piliformis was not found in this animal.