A series of laboratory experiments was conducted to study one of the possible explanations why the endoparasitic hyperparasite,Alloxysta victrix(Westwood), was numerically more abundant by a 2:1 ratio in previous field studies over the ectoparasitic hyperparasite,Asaphes californicusGirault.It was hypothesized that perhaps a time factor or the day of attack in reference to the developmental stage of theAlloxystalarva/pupa within the aphid mummy could influence the competitive success or failure of the femaleAsaphesin her attempt at hyperparasitization as a tertiary. The host aphid was the pea aphid,Acyrthosiphon pisum(Harris), and the primary parasite wasAphidius smithiSharma&Subba Rao. The developmental time of the 1st hyperparasite,Alloxysta,inside the aphid mummy was long enough to permit 11 test-days for attack by the 2nd hyperparasite,Asaphes.Results of these laboratory experiments on hyperparasite competition indicated that 89.3% of the hyperparasites which emerged from mummies containingAlloxystaand attacked byAsaphesduring the 1st half of the test-period (Days 1–6) wereAsaphes,and only 10.7% wereAlloxysta.However, during the 2nd half of the test-period (Days 7–11), successful hyperparasitization byAsaphesdecreased to 33.4%, and conversely 66.6% of theAlloxystasurvived this attack and emerged as adults.Thus, the ectoparasiticAsaphesdoes not invariably dominate the endoparasiticAlloxystawhen the 2 hyperparasites come into direct association, as had been supposed. The numerical abundance and apparent competitive superiority ofAlloxystaoverAsaphesin previous field studies is due not merely to its greater specificity and higher searching efficiency, but to its ability to compete successfully withAsaphesby increasingly surviving attack the later this occurs during its development inside the mummy.