The aspen leaf miner, Phyllocnistis populiella Cham., has recently become common and very abundant in western North America, and has been authoritatively identified only from trembling aspen, Populus tremuloides Michx. Overwintered adults lay eggs on young aspen leaves in spring, and new-generation moths emerge in about two months. Only the single epidermal cell layer is mined on the upper or lower leaf surface, most of the feeding being done by the third-instar larva in about a week. Heavy attack results in defoliation by mid-summer. Activities of breeding populations are affected by temperature (50-55°F. is optimal for copulation, 54-56°F. for feeding, and 52-58°F. for oviposirion) and developmental stage of aspen leaves. Eggs are laid singly near the apex of the leaf and tend to be evenly spaced. Distribution of eggs between leaves tends to be uniform in a particular level of the tree, although more eggs are laid in the lower crown. A female moth can develop more than 40 eggs, but only about 7 are usually laid because of limited opportunities. Mortality in the larval and pupal stages is often high; population decline usually follows mortalities above 70%. Parasitism sometimes plays an important role, but the independence of population changes between broad geographic areas suggests that numbers of P. populiella may be strongly influenced by climatic factors. Population trends and damage can be assessed conveniently for large geographic areas and for specific sites by continuing studies in permanent sample plots. Expected damage may be predicted from estimates of new-generation adult populations. Because inter-tree variation exceeds intra-tree variation, more trees and fewer branches per tree should be taken to increase sampling efficiency. Also, greater precision is attained by the use of the individual leaf surface rather than the leaf as a basic sample unit.