The aphid parasite,Aphelinus perpallidus(Gahan), was exposed to different densities, (0.125, 0.25, 0.50, 0.75, and 1.0 aphids per cm2)of its host,Monellia caryella(Fitch). Aphid density and fecundity of unmated and matedA. perpalliduswere significantly related. Mated females produced both male and female offspring, whereas unmated females produced only males. Adult parasite emergence was not significantly affected by host density. There was no significant difference in emergence between mated and unmated females. Significantly more male parasites emerged at all densities. Parental age of unmated or matedA. perpallidusfemales did not significantly affect parasite fecundity or emergence. Fecundity and emergence were significantly higher with unmated females. Mated females produced significantly more male progeny. These data indicate that matedA. perpallidusfemales from 0 to 14 d old must be used and exposed to aphid density levels of 0.50–0.75 aphids per cm2to obtain enough parasitism in order to have higher production of female progeny.