Recently Bombus terrestris has invaded into the peninsulas of eastern Hokkaido, northern Japan, where the restricted distribution range of the rare native species B. florilegus is located. To demonstrate the establishment status and flower use of B. terrestris, we investigated flower visits by bumblebees in 2007 in the Notsuke Peninsula. A total of 554 introduced and native bumblebees foraging mainly on native plants were recorded and B. terrestris foraged more frequently native plants than non-native plants. This species however was much less abundant, comprising only 2.5% of all observations. At the time of excavation, a feral B. terrestris nest found during the survey yielded 362 worker cocoons but no sexuals. Because the foundress was still alive and much food was stored in the nest, this colony likely would have been able to reproduce successfully if not excavated. Our results indicate successful naturalization of B. terrestris in the native grassland vegetation and the species has potential to negatively affect native bumblebee species.