The morphology of lymph node tissue from normal hamsters and from hamsters experimentally infected withTreponema pertenueGauthier was compared by means of light and electron microscopy. The capsules of the lymph nodes from infected hamsters showed an increased thickness in comparison with those of the non‐infected animals. The infected lymph nodes differed from normal lymph nodes by small accumulations of neutrophilic leucocytes in the cortical areas. In addition, the amount of intercellular collagenous matrix present between large elongated cells was greatly increased in lymph nodes from infected animals. Electron microscopy of thin sections of infected lymph nodes showed intercellularly located treponemes in the leucocyte infiltration areas. These regions also showed the increased amounts of the collagenous matrix. Treponemes were occasionally found intracellularly in macrophages. These treponemes did not show their typically helical shape, but were present as spherical forms or cyst