The life history, behaviour, and mortality of the western ash bark beetle, Hylesinus californicus (Swaine), was investigated in green ash, Fraxinus pennsylvanicus var. subintergerrima (Vahl) Fern., in Calgary, Alberta. This species is univoltine with an apparent obligate diapause in the adult (overwintering) stage. Adults emerge from overwintering sites in the bases of ash boles from mid-April to mid-May and fly or walk up the bole to the crown where maturation feeding, mating, and reproduction occur. Eggs are present by late April, larvae by early May, pupae by mid-July, and new adults by late July. There are three larval instars. Beetles emerge from larval galleries in August and enter fresh phloem in uninfested parts of branches to feed for up to 6 weeks. Beetles leave feeding galleries from mid-September to early November and migrate by walking, falling, and flying to overwintering sites in the base of ash boles. Total mortality from egg to eclosed brood adult was 80.6%. Disease, parasitoids, and dehydration were the main causes of mortality. Eight hymenopterous parasitoid species were identified; the pteromalid wasp Rhaphitelus maculatus Walker was the most abundant. Tree hazard rating, maintenance of tree health, removal of susceptible and infested host material, and application of pesticides are suggested for management of western ash bark beetle populations.