In greenhouse and cage tests, the size of colonies of Acyrthosiphon pisum (Harris) (Hemiptera: Aphididae) was greatly reduced through decreased fecundity and increased mortality and dispersal rates caused by parasitization and harassment of the aphids by the aphidiid parasite Aphidius smithi Sharma & Subba Rao. In the greenhouse, undisturbed colonies increased 26% in 24 hr, where the adult aphids were retained, but decreased 3% where they were removed. Where parasites were introduced, the colonies were reduced by more than 33%. The reduction in colony size in both greenhouses and cages was chiefly the result of the physiological effects of parasitization on the fecundity of the newly molted adult aphids. Harassment also caused the aphids to leave the host plant and thus increased mortality, because even the disturbance of a neighboring aphid by a parasite was enough to cause the aphids to leave the plant. Even female parasites that lacked the ability actually to parasitize the aphids were able to harass them and significantly to reduce the size of the colonies. Aphids dispersed by the parasites distributed themselves randomly, while only excess individuals moved from undisturbed colonies and these individuals went only as far as the nearest host plant.