Nuclear-polyhedrosis virus, introduced into virus-free populations of the forest tent caterpillar, Malacosoma disstria (Hübner), in 1963, was carried over into the 1964 host generation and was spread into the surrounding areas from the points of introduction. The extent of spread in 1964 appeared to be influenced by the size of the area treated in 1963. In 1964 the epizootics developed slowly during the early instars and reached the highest levels late in the last instar. Many infected larvae crawled to the tops of the trees and died while eggs were being laid, thus, some egg masses were contaminated. This is one means by which virus may be spread from one generation to the next, but egg-transmission by adults appears to be the main method of spread over wide areas. Adults of the dipterous parasite, Sarcophaga aldrichi Park, may be important dispersal agents of disease in localized areas because they feed on virus-diseased larvae and are present from the time larvae enter the third instar until after adults have oviposited.