The aim of this study was to compare the genomes of two strains of intestinal treponemes, which were isolated from patients suffering from intestinal disorders, with that of Treponema hyodysenteriae, the known etiological agent of swine dysentery (bloody scours). The guanine-plus-cytosine contents of the three DNAs were found to be 28.5 to 30.0%. DNA-DNA hybridization in liquid phase indicated a high degree of homology (56 to 95%) among the human strains and with T. hyodysenteriae. One of the human strains in particular displayed a very high homology (91 to 95%) with T. hyodysenteriae. The overall conclusion is that treponemal strains pathogenic for humans and animals are clustered within the same species (we propose T. hyodysenteriae), which suggests the possibility of exchange of pathogenic microorganisms between domestic animals and humans.