1. A method for the pure cultivation of Spirochœta duttoni, Spirochœta kochi, Spirochœta obermeieri, and Spirochœta novyi is described in this paper. In vitro these strains reach their maximum growth after seven, eight, or nine days at 37°C. For their multiplication they require the presence of a piece of fresh sterile tissue and a body fluid capable of forming a loose fibrin with the tissue. The presence of some oxygen seems indispensable for their growth, since they fail to grow in an atmosphere of hydrogen or in vacuo. No growth was obtained at room temperature. 2. From cultures that show a good growth subcultures can be made and the growth can be kept up in this way for many passages. 3. The pathogenicity of these organisms is not lost by cultivation, although there is a tendency for the virulence to become attenuated after growth in vitro has continued for a long time. 4. Longitudinal division has been observed and was followed under the dark-field microscope in fresh preparations from cultures. It occurs in all the species irrespective of the length of the organism. Transverse division seems also to occur, but I have not yet observed the entire process.