Experimental infections with Yersinia pestis were followed in groups of rock squirrels. Development of coagulopathy and pneumonia were observed in 2–4% and 11–12% of the test animals, respectively. Susceptibility to experimental infection was heterogeneous with some animals surviving inoculation with large numbers of organisms and others succumbing after inoculation with small numbers. Production and longevity of serum antibody titers, as measured by passive hemagglutination tests, were variable as well, and apparently unrelated to dose. The data presented attest to the need for care in interpreting serologic test results for individual animals.