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We investigated the cause of extraordinarily small body mass in Norway rats (Rattus norvegicus) living in the Ogasawara (Bonin) Islands in the subtropical climate zone in Japan. We compared body masses of Norway rats living in four localities, the Hahajima group of the Ogasawara Islands, uninhabited islands in Hokkaido in the subarctic climate zone, a business district in Yokohama, and an artificial islet in Tokyo Bay in the temperate climate zone. Regressions of body mass and age (in months; estimated from lens weight) showed that weights of Norway rats on the Hahajima Islands were about half the weights of rats in the other three localities. Crown length of the maxillary molar row was similar in three localities (Hahajima, Hokkaido, and Yokohama), and both the head–body length and the tail length were similar in Hahajima and Hokkaido, suggesting that the low body mass of the Hahajima rats was due to environmental factors rather than genetic factors. Stomach contents of Norway rats on the Hahajima Islands were predominantly (95.2% by vol.) plant matter, which is not the usual food preference for the species. We hypothesize that a low-protein diet restricts body mass of Norway rats on the Ogasawara Islands.