Abstract: The activity of the microbial insecticide, Bacillus thuringiensis (B.t.) Berliner has been found to be synergistically enhanced by the addition of unsaturated 18-carbon fatty acid salts. A synergistic effect with respect to increased larval mortality of the western spruce budworm, Choristoncura occidentalis Freeman was obtained in the C.sub.18 unsaturated fatty acid/B.t. treatments. Enhanced mortality was also observed in C.sub.18 fatty acid/B.t. treatments of the larval form of the silverspotted tiger moth, Halisidota argentata Packard. The increased insecticidal activity of the entomogenous bacteria, Bacillus thuringiensis (B.t.) Berliner by the addition of unsaturated 18-carbon fatty acids represents a significant breakthrough for the expanded use of this microbial insecticide.