Epizootiological studies of hantavirus infection among urban rats were carried out through the surveys repeated 11 times at the same dumping ground area in 1983 to 1988. A total of 279 rats (Rattus norvegicus) were captured during the surveys. Sero-positive animals to hantavirus strain SR-11 were detected in all the surveys. Overall positive rate of rats 6 months old or more (94/128, 73.4%) was significantly higher than that of younger rats (23/151, 15.2%, x2 = 96.4, P < 0.001). Therefore, age dependent acquisition of hantavirus infection among rats was confirmed. Seven hantavirus strains, KI-83-262 (August, in 1983, designated as strain KI-262 in our previous report (2)), KI-85-1 and 85-2 (July in 1985), KI-88-4, 88-11, 88-15 and 88-24 (October, 1988) were isolated from lung tissues of adult rats which have high titers of neutralizing antibody. Although the serum specimens of virus carrier rats neutralized the infectivity of all the KI isolates, no apparent antigenic change in the isolates was detected by indirect immunofluorescent antibody (IFA) assay using polyclonal and monoclonal antibodies (MAbs) regardless of isolation years. However, neutralization test showed slight difference of antigenicity among KI strains. These results epizootiologically confirmed that hantavirus infected persistently among urban rats in a presence of neutralizing antibody.