Bacillus thuringiensis var. kurstaki (B.t.) is the most widely used biological control agent against the spruce budworm, Choristoneura fumiferana (Clem.), a major defoliator of coniferous forests. The technology of applying the bacterium, however, is still not fully developed and the strategy of applying single or split applications of B.t. in unmixed stands is still in question. Double applications are generally accepted as necessary in mixed stands of white spruce, Picea glauca (Moench) Voss, red spruce, P. rubens Sarg., black spruce, P. mariana (Mill.) BSP, and balsam fir, Abies balsamea (L.) Mill., due to difference in the phenological development of the host trees and of the budworm infesting them. Field trials were conducted at Mine Centre, Ontario, to compare the efficacies of double and single applications of B.t. against the budwonn infesting balsam fir stands.