The spatial distribution of the woolly apple aphid,Eriosoma lanigerum(Hausmann), in a 41-yr-old apple orchard was analyzed using the negative binomialparameter k, Taylor's power law, and Iwao's regression technique. All three indices indicated a highly aggregated distribution of the aphid in the apple orchard. There was a weak but significant linear relationship between the negative binomial k and the mean density for the individual aphids but not for aphid colonies. Compared with Iwao'sregression, Taylor's power law provided a better description of the relationship between the variance and mean density. Woolly apple aphid occurred on the apple trees mainly as apterous virginoparae throughout the year and showed a preference for the lower part of the canopy and the trunk. At low infestations, the aphid is confined to the trunk and large branches but disperses to establish colonies on twigs or new lateral growths during peak populations. Woolly apple aphid has a limited ability to disperse between trees. Population density appears to increase mainly within trees. The relationship between the proportion of infested trees and mean aphid density was in the form of a rectangular hyperbola with an asymptotic upper limit of 33% infestation level.