The differences between R. prowazeki and R. mooseri have been, so far, discussed from their epidemiological traits, clinical signs, affinity for tunica vaginalis of guinea pigs or rats, affinity toward endothelial cells of blood vessels, structure of antigen, and antigenicity of complement fixation test, In recent time, the way of thinking about the differences between these rickettsiae tends to place great importance on complement fixation test in place of affinity toward tunica vaginalis or endothelial cells of blood vessels of guninea pigs. In order to clear the questionable points to differentiate the two types of rickettsia, the author has studied about the affinity of the rickettsia toward tunica vaginalis of guinea pigs and has considered again about the phenonena occurring in scrota of guinea pigs. Heart bloods of guinea pigs infected with R. prowazeki and R. mooseri were mixed, and this mixed blood was used as a starting-material for successive generation. The author found that by the successive generation to 10 generations through intraperitoneal route suiting to R. mooseri or through intracardial route suiting to R. prowazeki, the rickettsial strains of the original mixed blood could be isolated to each type of rickettsiae on the ground of complement fixation test about the isolated strains. It is not too much to say that scrotal phenomena caused by R. mooseri is, as expected, very important sign to differentiate R. mooseri from R. prowazeki.