Multiple, ligated loops of swine colon were used as an in vivo model in which to test enteropathogenicity of isolates of T. hyodysenteriae. Gross and microscopic lesions observed in 21 of 22 colonic loops in pigs killed 48-72 h after inoculation with isolates known to be enteropathogenic were characteristic of swine dysentery. These lesions were not observed in 12 loops exposed to uninoculated media or in 12 loops inoculated with nonpathogenic isolates of T. hyodysenteriae. The swine-colonic-loop technique provides a relatively rapid, economical, reliable model in which to test enteropathogenicity of T. hyodysenteriae isolates.