The following conclusions can be drawn from these observations: 1. The common rat flea (_Lœmopsylla cheopis_) prefers the rat to the guinea-pig. 2. In the absence of rats it will attack guinea-pigs rather than rabbits. 3. The fleas which have sucked blood from rats or guinea-pigs afflicted with plague septicæmia were found to harbor virulent plague bacilli inside of their bodies. 4. The transmission of plague infection by direct or indirect contact being excluded in our case, the fact that fleas of the same species and harboring plague bacilli were found on the rat and on the guinea-pigs, the presence of flea bites on the rats and on the guinea-pigs with positive findings of skin lesions on that part of the body where the fleas and flea bites were located, together with the anatomical picture of the findings in the guinea-pigs, lead to but one explanation; namely, that the plague infection was transmitted by fleas. III. OBSERVATIONS ON ANIMALS SUSPECTED OF PLAGUE Out of the several tens of thousands of rodents examined during the antirat campaign, we have found only two plague rats which showed the typical picture of natural plague infection in rat; that is, cervical buboes with surrounding œdema, subcutaneous injection, pleural effusion, enlarged spleen, and such changes of the liver as are characteristic of natural plague infection in rats. Microscopically, large numbers of plague bacilli were found in these cases, and pure cultures of _Bacillus pestis_ were recovered from the spleen. Histological examination of internal organs, particularly that of the liver, confirmed the bacteriological findings. The remainder of the plague rats exhibited only two of the signs of plague infection, namely, bubo and œdema of the surrounding tissue, and eventually hemorrhages. Besides plague infection, a great number of rats showed purulent conditions from causes other than plague. Abscesses of the lungs were frequently met with, and cervical or axillary buboes are not uncommon in Manila rats. Various pyogenic bacteria were found in the pus of such abscesses. Of the less common was _Bacillus pyocyaneus_ and the pneumobacillus of Friedländer. Chronic plague was excluded in these cases since the animal inoculation failed to produce plague infection.