The animals (other than controls) used in this experiment were trapped upon the U.S. Marine Hospital Reservation at Fort Stanton, New Mexico. After they had been kept in captivity for several weeks an effort was made to determine the susceptibility of the several species to infection with the plague bacillus. The culture of B. pestis used was one that had been isolated from the spleen of a naturally infected ground squirrel about two months prior to the present experiments. The third generation on agar was used, the culture having been grown for 72 hours at 370 C. In the case of the animals in which the culture was inoculated