Populations ofChromaphis juglandicola(Kaltenbach) and its natural enemies were assessed weekly during the growing season in 1969 and every 4 days in 1970. A parasite of the aphid,Trioxys pallidus(Haliday) (Aphidiidae: Hymenoptera), from Iran, was released and became established in the study area in 1969, but it did not affect the abundance of the aphid during that season. Large numbers of mummies overwintered, and the normal peak in aphid numbers in spring 1970 did not occur. Aphid numbers remained at levels about 0.01–0.001 of those in previous recorded years. An experimental release of aphids and use of a biological check method demonstrated that the parasite was responsible for the maintenance of the aphid at low numbers. The same tests showed that selective predation of parasitized aphids by the Argentine antlridomyrmex humilis(Mayr), presents a serious threat to continued control of aphid outbreaks by the parasite where ants are present. The rapid increase in the artificially induced outbreak of aphids indicated that it was not high temperatures that previously kept aphid numbers low in summer, but leaves made unacceptable by sooty mold and debris from the spring peak of aphid numbers.