The authors have described a new bacterium - Pasteurella intermedia, n. sp. obtained by inoculating a guinea-pig with 2 cc. of total blood from an individual who died from bronco-pneumonia and suspected of having a seriou form of neotropic exanthematic Typhus. They have impression that Pasteurella marsupialis and Pasteurella intermedia constitute a distinct and well-defined group among the Pasteurellas. Very small, having a great and persistent pathogenic power over the ususal laboratory animals, even when the specimens of the bacteria are preserved for years by replanting in common agar in the common temperature and illuminations of the laboratory. These two Pasteurellas, contrary to the others, have a great and unmistakable antigenic power for the formation of agglutinins and fixing of the complement and constantly give a testicular reaction in male guinea-pigs when the injection is intraperitoneal, high fever, constant and sometimes notable espelenmigalia causing confusion for an expermental and differential diagnosis with the VB race of neotropic exanthematic Typhus in Brasil (Disease of Pisa, Gomes and Mayer).