We investigated the prevalence of infestation by the bumblebee tracheal mite, Locustacarus buchneri (Acari: Podapolipidae), in four commercial colonies of Bombus terrestris (Hymenoptera: Apidae). All the foundresses, 91.8±5.67% (mean±SE) of worker bees, 73.9±6.85% of males, and 77.4±21.5% of new queens had the mite in their abdominal air sacs. The mean number of eggs, adult females, and mobile mites infesting a single bumblebee worker were 76.9±76.8 (range: 4–301, n=40), 3.78±3.45 (1–11, 40), and 3.45±2.98 (1–11, 20), respectively. From these values, we calculated the number of eggs per adult mite female to be 23.9±21.7 (range: 4–126, n=40). As a single worker bee had about 76 mites in her abdominal air sacs and approximately 90% of the bees in a colony of about 160 bees were infested by the mite, an infested B. terrestris colony has the potential to produce more than 10,000 L. buchneri individuals during colony development.