Isolates of Treponema hyodysenteriae from 25 geographically separated outbreaks of swine dysentery were tested for their ability to produce the disease. Clinical signs and lesions typical of acute swine dysentery were produced in 52 of 68 (75%) susceptible specific pathogen-free pigs that had been orally inoculated with pure cultures of 23 of 25 beta-hemolytic isolates. In addition, 13 weakly beta-hemolytic isolates of nondysentery origin with morphology similar to T. hyodysenteriae did not produce disease when orally inoculated into susceptible specific pathogen-free pigs. Two of these latter isolates, Puppy and B296, and one pathogenic, beta-hemolytic isolate failed to produce disease when orally inoculated into puppies.