At least 24 different serotypes were detected in populations of Borrelia hermsii that originated from a single organism. These serotypes were identified by staining with specific fluoresceinated antisera prepared against cloned populations of living organisms of each type. In the order of decreasing frequency, the 10 types more often encountered were 7, which was clearly dominant, and 2, 17, 24, 13, 2, 1, 21, 11, and 12. Each of the 24 types were shown to change to 7 or more other serotypes. Spirochetemia in mice was persistent, and relapses occurred when the concentration of organisms was sufficient for detection by visual means. After mice were inoculated with a single organism, peak spirochetemia usually occurred on day 4, after which clearance of organisms occurred, and an apparently pure population was replaced by a mixed population consisting of as many as seven variants. These types persisted for 2-3 d before being replaced by other types. Conversions occurred constantly and were independent of relapses. The rate of conversion in mice treated with cyclophosphamide to delay antibody production was comparable to that of controls. Spontaneous conversion was clearly demonstrated in tubes of fortified Kelly's medium inoculated with a single organism of type 7 or 21. 11 different variants appeared in eight cultures of type 21 by the time growth had reached 4 X 10(6)-10(7) organisms/ml. The rate of spontaneous change was estimated to be or approximately 10(-4)-10(-3) per cell per generation.