Two Norway rats, Rattus norvegicus, were found to be naturally infected with Echinococcus multilocularis in Japan. One of them was simultaneously infected with at least three different sized metacestodes of Taenia taeniaeformis. These two R. norvegicus rats and another R. norvegicus naturally infected with T. taeniaeformis and Capillaria hepatica were examined to see if they showed any antibody responses against these two cestode parasites with the view to obtaining more information on the importance of rats as the intermediate host for E. multilocularis. These R. norvegicus showed very poor antibody responses against the two cestode species, although the Wistar rats, R. rattus, experimentally infected with a single smaller sized metacestode of T. taeniaeformis showed stronger responses not only against T. taeniaeformis but also against E. multilocularis. Therefore the three R. norvegicus naturally infected with E. multilocularis and/or T. taeniaeformis demonstrated virtually no immune response, at least against these cestodes.