Two days after the death of rat 2 three guinea-pigs, which were kept in separate cages in the same room, were found dead of plague (smears and cultures were both positive). Several fleas (_Lœmopsylla cheopis_) were found on the necks of these animals. They were collected and inoculated in the same way as the fleas from the first rat. The experimental animal, which was inoculated with the fleas, was killed and found to be infected with plague. The findings were local reaction, inguinal buboes, and typical spleen. Smears and cultures were positive for _Bacillus pestis_. Although numerous healthy guinea-pigs were examined in the same plague house, no fleas could be found at that time, only the 2 rats and the first 3 guinea-pigs are positively known to have harbored fleas, the latter after the death of the rats and not before. The gross lesions in these naturally infected guinea-pigs were somewhat unlike those found in guinea-pigs infected either by vaccination or by intraperitoneal or subcutaneous inoculation. All except one showed primary buboes on the neck with more or less extensive hemorrhagic œdema extending in some cases over the thorax. There was little pleural effusion present; the spleen always showed typical changes of necrotic foci varying in size and number. In one instance similar foci were found also in the liver, large enough to be visible macroscopically. This was in a case where like changes were found in the lungs. Only one of the guinea-pigs showed an exception, in that the primary buboes were located in the inguinal region, with pelvic and axillary glands secondarily involved. These are the findings usually met within guinea-pigs artificially infected with plague by the vaccination method, if the lower part of the abdomen be chosen for inoculation. The reason for such a deviation from the findings in the rest of the guinea-pigs may lie in the fact that this animal was almost completely deprived of hair by a skin disease. It is of importance to mention the skin lesions which were found on the necks of the guinea-pigs, particularly under the chin. Besides small red spots which appeared to be fresh flea bites, small, elevated, and fairly deep infiltrations partly covered with moist scab were found in the skin under the chin. Other animals showed changes usually found in the scarified skin of guinea-pigs after artificial inoculation with plague material. The base of each cutaneous efflorescence was hemorrhagic and œdematous. A histological study of the tissues of these guinea-pigs known to be naturally infected by plague fleas showed the following changes: