1. Squirrels and chipmunks were found to be susceptible to infection with the Spironema recurrentis of North China, but no relapses occurred in normal animals. 2. Splenectomy caused an increase in the intensity of the infection and the appearance of one or two relapses. 3. By inoculating splenectomized squirrels with a single human strain of spirochetes, six different strains were produced as a result of relapses. 4. The strains all retained their specific agglutinating characteristics during the period of observation. The oldest strain was observed for 132 days and through 38 transfers, the strain last isolated for 22 days and through 6 transfers. 5. The sequence of strains in relapses was not always the same. Sometimes new strains were produced, at other times there was reversion to an older strain. 6. Certain relationships between strains were established. The related strains were always those which appeared, or theoretically might have appeared, in alternate attacks. In only one case did related strains appear in two consecutive attacks. 7. Squirrels whose spleens were not removed had only one attack and developed immune substances only against the strain inoculated or against closely related strains. It was possible by reinoculation to produce infection in such squirrels with as many as four different strains in succession. 8. The spleen has a protective influence against the development of the relapse in the squirrel, but apparently does not control the formation of specific immune substances. 9. An explanation of the clinical course of relapsing fever is suggested on the basis of the observations recorded. 10. There is, at present, no justification for the division of the spirochetes of relapsing fever into different species.