The author recapitulates the data already known on the filtration of many Rickttsias and on what he calls the evolutive cycle of the same, citing the works of S. B. Dulky and E. E. Gordon on the Coxiella papilliae and the observation of A. Donatien and F. Letoquard as also those of P. Giraud. He refers to the filtration of the agent of "Q" fever. He then alludes to the works of Mme. Ruth Rien Gutfeund on the finding of Rickettsia Prowazeki in domestic animals in Ethiopia and their transmission by ticks, saying that this confirms his own ideas, expressed long ago, that there is not, in the exanthematic typhus group, usually, strict specificness for the carries, therefore this feature cannot serve as a basis for rational classification. He describes a clinic case, showing the difficulty of a diagnosis confounded with typhoid fever, even after long experience in examining the disease and that only an inoculation in sensitive animals - here guiena-pigs - can decide the question. He next studies two epidemical outbreaks of neotropic exanthematic typhus - in 1950 in Carmópolis (Minas Gerais); another in 1956 in Mucuri (Bahia). In both the percentage of deaths, in the serous cases, that had no treatment, was high and the therapeutics by the antibiotics, principally Cloromicetina and Terramycina was brilliant. In these two epidemical outbreaks as happened in 1941 (4 cases in a house) and 1948 (6 cases in a house) there were 2 and 3 cases in a family. the reactions of the Complement fixation (R. F. C.), Weil-Felix (W. F.) and Widal confirm the diagnosis if neotropic exanthematic typhus.