1. Pleomorphic, bacterium-like, Gram-negative, intracellular microorganisms, which stained much less intensely with ordinary dyes than most bacteria were found in sixteen species of ticks comprising examples of the Argasidæ and the Ixodidæ.
2. In six of these species studied intensively slight differences in the microorganisms were detected, sufficient to permit identification of the vectors by microscopic examination of the microorganism alone.
3. No evidence was seen of injury to the tissues of the arachnid hosts of the microorganisms other than that incident to mechanical distention of the cells containing them.
4. The detection of the microorganisms in the eggs of ten species, in the unfed larvæ of eight species, and at nearly related stages throughout the life cycle of three others leads to the conclusion that they are transmitted hereditarily.