Serologic evidence was used to investigate the spatial and temporal distribution and host range of hantaviruses in small mammal communities in Baltimore, MD. Immunofluorescent antibody (IFA) reactive to a Baltimore rat isolate of Seoul virus was detected in 44% of 404 Norway rats captured at 4 park or residential locations; 21% of 121 meadow voles captured at 4 park locations possessed significant IFA titers to Prospect Hill virus. Evidence from plaque neutralization assay of rodent sera suggested that Seoul virus and Prospect Hill virus circulated concurrently in voles and rats, respectively, at 1 park. No cross infection of virus between these primary reservoir species was observed, as determined by higher specific neutralizing antibody titers to the characteristic virus for each host species. Only 4% of 449 house mice and 1% of 94 white-footed mice captured at the same sites as the primary host species were seropositive to hantaviruses, despite extensive demonstrated overlap in time and space with the primary host species.