Serologic epidemiology was applied to investigations of wild rodent plague in areas of California, Arizona, Colorado, and Utah. Results indicated that the passive hemagglutination test can be of value in detecting antibodies to the water-soluble Fraction Ib antigen of Pasteurella pestis in wild rodent or other mammalian sera. Positive tests were obtained with sera of mammals from all areas with a concurrent or recent history of wild rodent plague. Thus, the current status of P. pestis infection in mammals was determined, as well as a retrospective determination of past infection. Some evidence was found suggesting that certain plague-resistant rodents that have a wide geographic distribution (i.e., Peromyscus maniculatus) may serve as “indicator animals” in plague investigations.