The spruce budworm, Choristoneura fumiferana (Clemens), is susceptible to a nuclear polyhedrosis virus and to a granulosis virus which may occur as single infections (Bergold 1950, 1951) or as double infections (Bird, 1959). Laboratory studies have shown that relatively heavy concentrations of either virus musr be injected or fed to hudworm larvae to cause infection and death. In one quantitative study Bergold (1951) estimated that the intralymphal LD50 of the polyhedrosis virus for the budworm is about 5000 times that for the silkworm, Bombyx mori L., when each is administered to its natural host. Field tests of the viruses were made in 1959 and 1960 to determine whether infection and mortality would result from spraying suspensions in infested forests.