The hemlock woolly adelgid, Adelges tsugae Annand, is maintained at innocuous population levels in Japan on its 2 native hosts, Tsuga diversifolia Masters and T. sieboldii Carriere, by host resistance and natural enemies. The most common enemy associated with infestations of A. tsugae in Japan was Diapterobates humeralis (Hermann), an arboreal oribatid mite that inhabits coniferous forests throughout the Northern Hemisphere, where it usually feeds on decaying plant tissues, fungi, algae, and lichens. This mite occurred in 12 of 13 prefectures and at 42 of the 76 sites (55%) that were sampled in Honshu, Japan, from 34 to 37° N latitude and between sea level and 2,100 m elevation. This included 17 of 37 forest sites (46%) and 23 of 37 ornamental sites (62%) where adelgids were present, and 2 uninfested ornamental sites. Laboratory cage experiments revealed that D. humeralis did not eat adelgid eggs or nymphs, but rather consumed the woolly filaments that enveloped the ovisacs. However, in so doing 20 adult mites dislodged >95% of the eggs contained within 10 ovisacs from hemlock branches in 7 d. This 2:1 ratio of mites to ovisacs is representative of densities in the field. More eggs were dislodged from ovisacs located along the twig (99%) than from those located at the base of terminal buds (95%), the usual oviposition sites when adelgid densities are high and low, respectively. In a field experiment that simulated effects of egg dislodgment by mites, not a single adelgid was found on any of 4 hemlocks beneath which 2,383 eggs had been placed 16 d earlier. Hatching adelgid nymphs were unable to colonize nearby trees and fell victim to desiccation and generalist predators, mainly ants and spiders that inhabited the forest floor. During the same period 96% of 2,262 adelgids placed directly on 4 other hemlocks successfully colonized the trees. D. humeralis destroyed 86–94% of adelgid egg masses at all 17 infested forest sites and 99–100% of them at 16 of the 23 infested ornamental sites. Included in this latter group were 2 sites where adelgids had been controlled on T. canadensis (L.) Carriere, a highly susceptible North American species. At the other 7 infested ornamental sites where adelgid mortality ranged from 55 to 83%, mites were abundant and were ravaging egg masses of A. tsugae when trees were 1st sampled. At 2 of these latter sites that were revisited 3 wk later, only 2–3% of the ovisacs were viable and adelgid densities had been reduced by 90–93%. Although D. humeralis was not a predator of A. tsugae , it was a highly effective control agent by dislodging adelgids from the trees and killing them as it fed on the woolly filaments surrounding the ovisacs. The wide climatic and environmental tolerance of this mite in Japan, its voracity in consuming the woolly ovisacs of A. tsugae , its propensity to dislodge adelgid eggs from trees, its ability to reduce adelgid populations to innocuous levels on T. canadensis in Japan, and its harmlessness to other organisms in the environment make D. humeralis an excellent biological control candidate for North America.