Seroepidemiological studies of hemorrhagic fever with renal syndrome (HFRS) virus infection were carried out among urban rats (Rattus norvegicus and Rattus rattus) and small field rodents in Hokkaido, Japan. An urban rat colony that was seropositive to SR-11 strain of HFRS virus (laboratory rat origin) was demonstrated in February 1983 at a dumping ground area of Kami-iso Town near Hakodate port. An HFRS-related virus, named KI-262 strain, was isolated from the lung tissue of a seropositive rat using Vero-E6 cell culture. Antigenicity of the isolate was closely related to Hantaan 76-118 and SR-11 strains by the indirect immunofluorescent antibody (IFA) test. No seropositive rat was found among the 861 rats captured in 38 other regions. It is unclear whether or not the infected rats in the positive area were introduced from abroad, though the area is located near Hakodate International Port. Furthermore, higher positive rates of urban rats in the Kami-iso area were observed in the spring and winter than in the summer and fall. Significantly high proportion of positive cases was observed among adult rats (six months or older) than among younger animals. The seasonal and age distribution of positive cases suggested that the virus was not readily transmitted from one infected rat to another. One seropositive case of a small field mouse (Clethrionomys rufocanus bedfordiae) was detected around the Kami-iso area.