Wild rodents involved in a plague epizootic were trapped on a bimonthly schedule at 15 trap sites distributed throughout the San Bruno Mountain plague pocket located in northern San Mateo County, California. The percentage of positive sera obtained from Microtus californicus varied from zero in two sites in which Y. pestis had not been recovered from rodent flea or tissue pools to as high as 90% to 97% positives in Microtus trapped in four sites in which Y. pestis was recovered.

Analysis of the data available indicates that the rate of seropositive rodents, Peromyscus maniculutus and Microtus californicus, is correlated with gross numbers of fleas found per trapline.