The biology of Alniphagus aspericollis (LeConte) was studied in thinleaf alder (Alnus tenuifolia Nutt.) in northern Idaho. Infested alders were mature and dying from infection by a white rot fungus, Phellinus igniarius (L.) Quél. The beetle is monogamous and has one generation per year, with broods overlapping in developmental stages throughout summer. Alniphagus aspercollis individuals passed winter as larvae and sexually immature brood adults, some of which overwintered in short hibernation galleries. Beetles emerged in May to create a new generation. Egg galleries were short, with closely spaced, elongate larval mines. Larvae passed through 4 instars before pupating. New records of insect predators and commensals associated with A. aspericollis are presented. Features of the smaller Alniphagus hirsutus Schedl in Alnus sinuata (Regel) Rydb. from earlier Idaho collections are compared to A. aspericollis, and the published listing of A. tenuifolia as a host of A. hirsutus is questioned.