These studies were initiated to determine the prevalence and hosts of hantaviruses within the rodent population of Nevada and northeastern California. A total of 1,867 rodents were collected, sexed, weighed, identified, and tested by enzyme-linked immunosorbent assay for the presence of antibody against hantavirus nucleocapsid. The primary hosts for hantaviruses in this region were found within the family Muridae (Peromyscus maniculatus. Reithrodontomys megalotis. and Microtus montanus). Studies over time of animals within a defined geographic area indicated that animals with hantavirus antibody are not distributed uniformly over the rodent population in a specific area but were found in foci spanning a distance of only several hundred meters. The antibody prevalence in a given geographic area remained relatively constant with repeated sampling of between 0% and 30%. These data support the hypothesis that rodents within the family Muridae are the primary reservoir for hantaviruses, and the primary risk to biologists for exposure to hantavirus is by contact with members of this family.