Rats trapped from 14 locations in Baltimore, Maryland, were shown to have antibody to Hantavirus. Antibody prevalence rates were higher in residential locations than in parks. Infected rats were obtained continuously over a six-year period from 1980-1985 at five locations, indicating the enzootic nature of this infection in urban rats in the United States. Prevalence of antibody and geometric mean titers increased with rat mass and sexual maturity, suggesting that infections in rats are acquired through age-related mechanisms. Three isolates of Hantavirus were obtained from rats. One of these isolates was shown by serologic tests and monoclonal antibody reactivity patterns to be similar, if not identical, to rat virus previously obtained from Philadelphia. The presence of rats in many urban settings and the high prevalence of Hantavirus infection in this species suggest that human exposure to this virus is occurring in the United States, although human disease from this exposure has not been recognized.