The microhabitat distribution and spatial dispersion patterns of walnut aphid eggs and viviparae were assessed in a commercial walnut orchard in Tracy, Calif., during 1977 and 1978. Significantly higher egg densities were found on the smaller peripheral branches of walnut trees (1–2 cm diam) than on larger branches. Few eggs were found on extremely small twigs (ca. 0.55 cm diam) from the previous season's growth. Significant differences in egg density were sometimes observed between top and bottom regions and among cardinal quadrants of the tree. No significant differences in egg density were observed among tree. The spatial dispersion patterns determined for walnut aphid eggs were generally aggregated except at extremely low densities where the pattern was random. No significant differences in aphid density were observed among cardinal quadrants of the trees and only occasional differences were observed between top and bottom regions of the trees. Substantial variation in aphid density was observed among basal, middle, and terminal regions of the trees. Few significant differences in aphid density were observed among trees and none was observed among compound leaves. Significantly higher aphid densities were found on the middle and distal leaflets of compound leaves. Within-tree, among-tree, and among-compound leaf distribution patterns showed that the aphids were generally aggregated.