The introduced Bombus terrestris has recently become naturalized in Japan and it may be responsible for the observed decline of native bumblebee populations. We compared the foraging ability of B. terrestris and a Japanese native bumblebee, B. ignitus, in an experimental cage. The 6-day experiment showed no significant difference in mean foraging load between them even though B. terrestris was significantly smaller than B. ignitus. However, B. terrestris was significantly more efficient, with a higher mean foraging load per unit time, and also brought back more forage per unit body mass than B. ignitus. The proboscis length of B. terrestris workers was more suited to the flower size of Salvia farinacea and the species may thus be more efficient in foraging. Once it has invaded a new region, B. terrestris can become the dominant bumblebee. When resources are low, B. terrestris, with a large number of superior foragers, can disadvantage native bees through resource depletion.