Transmission experiments with Treponema hyodysenteriae were conducted with mice and pigs, using laboratory mice as the carrier host. Mice were intragastrically inoculated with T hyodysenteriae and placed in a contact with nonexposed mice or healthy pigs. Contact mice mingled with infected mice at 5 or 180 days after inoculation were shedding T hyodysenteriae in their feces 9 to 14 days after exposure. Pigs exposed directly to infected mouse feces were shedding T hyodysenteriae in their feces after 5 to 17 days of exposure and developed swine dysentery after 11 to 13 days. Transmission of T hyodysenteriae to pigs through direct contact with feces from infected mice indicates that rodents in the field may be a reservoir host and may also be involved in spreading swine dysentery.