Technical Abstract: Many organisms introduced into a novel environment fail to establish. One underlying process is the Allee effect, i.e., low survival and reproduction at low population densities. Although observations on the positive relation between initial population size and establishment probability suggest that Allee effects could be a widespread in biological invasions, experimental demonstrations are scarce. Here, we used the biological control program against DIURAPHIS NOXIA (Hemiptera: Aphididae) in the United States to manipulate initial population size of the parasitoid APHELINUS ASYCHIS (Hymenoptera: Aphelinidae) introduced from France. For eight populations and three generations after introduction, we measured spatial distribution and spread, density, mate-finding, and population growth. Dispersal depended on density in the first generation, but smaller initial population size resulted in lower density in all generations studied. The proportion of mated females and the population sex ratio were not affected by initial population size or population density. Net reproductive rate decreased with increasing density for each generation, but for a given density, net reproductive rate was smaller in populations initiated with few individuals than in populations initiated with many individuals. Hence our results revealed a demographic Allee effect. Mate-finding was excluded as an underlying mechanism, and other component Allee effects where probably overwhelmed by negative density-dependence. Rather, the continuing effect of initial population size on population growth suggests genetic processes underlying this demographic Allee effect.