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A commercialized pollinator introduced from Europe, Bombus terrestris, has colonized Japan. We investigated nest density and genetic structure in two sites based on worker genotypes at 12 microsatellite loci. We confirmed that five workers were triploids using multilocus genotypes and flow cytometry, indicating that queens mated with diploid males and produced triploid workers. The inbreeding coefficient of diploid workers representing individual colonies was significantly positive (FIS= 0.048) in a site where triploids were found. Genetic diversity in the sites was as high as that in native regions in Europe, and genetic differentiation between the sites was low (FST= 0.006). The maximum distance between sampling locations of full-sib worker pairs indicated that the radius of a foraging range was at least 782 m. The estimates of nest density were 31 and 89?km-2in the two sites, suggesting that the nest density in a colonized region can be higher than that in the native regions. [ABSTRACT FROM AUTHOR]