The biology of the willow bark beetle, Trypophloeus striatulus (Mannerheim) was studied in its primary host, Salix alaxensis (Andersson) Coville, at 28 locations in Alaska, and in the laboratory. Most broods transformed to adults before fall, emerged, and excavated solitary chambers in the bark, where they passed over-winter before becoming sexually mature in spring. Emerged adults walked on the bark for protracted periods, lacking any disposition to fly. This behavior contributes to reinfesting the stem, downward for several generations, thereby conserving a susceptible host (limited resource). A fungus, Cytospora sp., most likely chrysosperma (Person) Fries, was present in stems infested by the beetles. Adults usually chose a lenticel as a site for excavating a chamber, tapping the surface with their antennae, and were possibly attracted there by odor emitted by fungus-infected, underlying tissue. Egg galleries are cavelike. Eggs are exceptionally large relative to the adult, and are laid in a cluster averaging ≈20 per female. Larvae pass through three instars, feeding en masse in the first instar, then becoming solitary. Their mines contain almost entirely excrement, lacking fragments of phloem common in galleries of most bark beetles. Pupation occurs in a cell, excavated by the larva, which usually etches the xylem. A bird preys on over-wintering larvae; otherwise the beetle has few natural enemies. An unidentified endoparasitic nematode occurs in the hemocoele of adults. Commensals in galleries include several undescribed mites, and maggots of the dipterous family Sepsidae. The ecology of feltleaf willow and population dynamics of the beetle are discussed.