The development of three colonies of Bombus fervidus (Fabricius) and two of Bombus pennsylvanicus (Degeer) was studied. Population levels and the production of males and young queens varied among colonies and between years. Mean longevity of worker bees decreased significantly toward the end of the season. Mean size of emerging worker bees increased significantly with time. The decreased survivorship may be related to the seasonal size increase of the workers, which would result in an increased proportion of foragers. Age-specific life tables and survivorship curves were developed for workers within colonies, and for workers in cohorts based on emergence date. An increase in brood developmental times occurred near the end of the colony cycle. In the two B. pennsylvanicus colonies, oviposition of fertilized eggs decreased during the transition period from worker to queen production, and large numbers of male-producing eggs were laid. At the time of queen production a sudden increase in size of emerging females occurred. Neither a gradual nor a sudden change in larva/worker ratio appears sufficient to explain caste determination. Rates of parasitization of bumble bee queens at three sites in Southwestern Virginia were studied. Endoparasites of spring queens included Locustacarus buchneri (Stammer) (Acarina: Podapolipidae), a mite infesting the abdominal air sacs; the nematode Sphaerularia bombi Dufour (Tylenchida: Nematoda), and gregarious braconid larvae, probably Syntretus sp. (Hymenoptera: Braconidae). Ectoparasites included hypopi of the mite Kuzinia americana Delfinado and Baker (Acari: Acaridae) and Parasitus spp. mites (Acari: Parasitidae). / M.S.