Beetles of Trypophloeus populi attacked green bark of unhealthy aspen (Populus tremuloides) and hastened the death of the tree. Beetles of Procryphalus mucronatus favored dead bark and were of little significance in the death of the aspen. These two beetle species were distinguished from each other by their primary galleries, eggs, larvae, larval mines, and mating behavior. In T. populi, there were three larval instars and one to one and one-half generations per year; only larvae overwintered, and the eggs of the first generation were present by mid-July. In P. mucronatus, there were two larval instars and one and one-half to two generations per year; both larvae and adults overwintered, and the first eggs of the new season were present by late May.