Beginning with a spirochete-bearing O. turicata [female] reared from ticks collected in nature, the progeny resulting from the first oviposition by 1 [female] (in 1 instance 3 [female][female]) of each of the 5 succeeding generations were tested quantitatively for spirochetes. With exceptions noted, all tests were made by feeding the ticks individually, in each developmental stage, on white mice. The thick drop and Giemsa''s method of staining were used. Progeny found to be infective in each of the 5 generations amounted to 6 ticks (35%), 55 ticks (96%), 66 ticks (100%), 107 ticks (47%), and 136 ticks (100%), respectively. The invasiveness of the spirochetes did not diminish over a period of nearly 6 yrs. They appeared in the peripheral blood of all mice which were hosts to the F5 generation on the 1st day of examination, and the 5th day after feeding. These results indicate that the tick itself may be a more efficient "spirochetal reservoir" than the rodent host.