The interrelationship of an Iranian ecotype of Trioxys pallidas (Hal.) (Hymenoptera: Aphidiidae) and the walnut aphid, Chromaphis juglandicola (Kale), (Homoptera: Callaphididae), was assessed over a 4-year period at two localities in California. Limited additional data were obtained from other localities. Trioxys pallidas, a highly efficient parasite, which is biologically adapted to and phenologically synchronized with C. juglandicola, has brought about substantial biological control of this pest. Trioxys pallidus substantially dampens the aphid’s vernal oscillation, and normally restrains the amplitude of the summer and autumnal oscillations. Major economic benefits have been realized by the elimination of the aphid as a pest in springtime. Trioxys pallidus is at times heavily attacked by non-specific hyperparasites, but these, at most, hinder it but slightly. Certain insecticides can disrupt T. pallidus activity, permitting aphid outbreaks. Prolonged aphid scarcity, possibly abetted by hyperparasitism, also may cause a breakdown in parasite activity and a temporary aphid resurgence in midseason. During the 4 years of investigation, this occurred on one occasion in one of the study plots. More than one-half million dollars have accrued annually to the California walnut industry as a result of the C. juglandicola biological control program.