Three species of autoecious aphids, Callipterinella calliptera (Hartig), Euceraphis betulae (Koch), and Betulaphis brevipilosa Borner, commonly occur on European white birch trees, Betula pendula Roth, in northern California. Populations were sampled from 1981 through 1984 at two sites. Within-tree populations of all three species were more abundant in the lower and middle vertical thirds of tree canopies. Optimal sample sizes were estimated for individual trees as well as tree populations, given variable mean aphid densities, sampling reliability levels, and sampling costs. An aesthetic injury level for honeydew production was estimated. At both sites, all species exhibited spring and fall peaks with a summer decline in numbers. Callipterinella calliptera and B. brevipilosa populations increased during late summer while feeding on mature leaves. Euceraphis betulae preferred developing and senescing leaves and was reproductively inactive during summer. At the warmer inland site, C. calliptera and E. betulae were present only early in spring and late in fall and B. brevipilosa populations were virtually absent. Natural enemies associated with these aphids did not prevent the development of large populations, although coccinellids and syrphids demonstrated numerical responses. The only ant-tended aphid species, C. calliptera, developed the largest populations of all three aphid species.