We investigated introduced commercial colonies of the European bumblebee Bombus terrestris to determine whether they were infested by parasitic microorganisms. We isolated microsporidian spores from bees in a colony from the Netherlands. The shape of the spores and the number of polar filament coils, along with the developmental cycle and infection sites of the pathogen, were characteristic of Nosema bombi, a known serious pathogen of bumblebees. Laboratory studies showed that the isolated microsporidia were able to infect B. hypocrita and B. diversus, two native Japanese bumblebee species. These results strongly suggest that importing commercial colonies of B. terrestris carries the risk that pathogenic microsporidia will spread among Japanese bumblebee populations.