The collapse of the elm spanworm outbreak in Connecticut was caused primarily by anOoencyrtussp. previously determined asO. clisiocampae(Ashmead), but now thought to be an undescribed species. This parasitoid decimated the egg population in southwestern Connecticut in 1971 and in central and eastern Connecticut in 1972.In 1972 weekly collections of elm spanworm eggs from the infested area showed an increase in the amount of parasitism as summer progressed. At the western edge of the infestation, virtually all eggs were parasitized by July 25. Near the center of the infestation almost all eggs were parasitized by Sept. 5 and at the eastern edge of the infestation by Sept. 19. We attribute this eastward increase in part to the movevement of adult parasitoids into areas where they had previously been absent or rare.Superparasitism of elm spanworm eggs was common in dense populations of the parasitoid. At sites where parasitism was high and superparasitism prevalent, many host eggs were dead and were supporting growth of fungi.