Serologic testing of badgers (Taxidea taxus) was used to monitor plague (Yersinia pestis) in a Townsend ground squirrel (Spermophilus townsendi) population in 10,000 ha of the Snake River Birds of Prey Study Area, Idaho. Eighty-six percent of the 294 sera tested in 1975 and in 1976 were positive. Significantly fewer (72%) seropositives occurred in 1977. Seasonal changes in the percentage of seropositives and the decline in 1977 were probably due to the phenology of the Townsend ground squirrel and the proportion of that species in the badger's diet. Eight Townsend ground squirrels found dead had positive bacteriologic tests for plague; however, a high mortality in the ground squirrel population was not observed. Food habits and movement patterns of badgers made them ideal for documenting the geographical and temporal characteristics of the plague focus.