The fluorogenic 5′ nuclease polymerase chain reaction (TaqMan) assay, which was developed in this laboratory, was further optimized and used to screen hogs for Yersinia enterocolitica (4). Tonsil samples (n = 1,647) collected during the National Animal Health Monitoring System (NAHMS) Swine 2001 program were tested for the presence of the ail gene, which is present in virulent Y. enterocolitca. Tonsil swabs were collected on farms (n = 101) located in Arkansas, Colorado, Illinois, Indiana, Iowa, Kansas, Michigan, Minnesota, Missouri, Nebraska, North Carolina, Ohio, Oklahoma, Pennsylvania, South Dakota, Texas and Wisconsin. Of the 1,647 tonsil samples, 11.4% (n = 188) were positive.