A seroepidemiological survey of hemorrhagic fever with renal syndrome (HFRS) virus infection in 403 urban rats (Rattus norvegicus) captured on two different islands in Tokyo Bay was carried out using the indirect immunofluorescent antibody (IFA) test with the SR-11 strain of HFRS virus. Antibody positive sera were detected in 99 out of 355 (27.9%) rats from the island with reclaimed ground area, but none were detected in 48 rats from the other islands. IFA titers of the positive sera varied from 16 to 4,096 (mostly 128) with a peak at 1 : 128, and antibody positive ratios were significantly higher in the small rats (less than 50 g, probably of younger age) than in the large ones. A causative agent antigenically related to HFRS virus was isolated from lung tissue of seropositive rat using Vero-E6 cells. These results revealed the existence of HFRS related virus among urban rats in the coast of Tokyo Bay, Japan.