The microbial insecticide, Bacillus thuringensis (Bt), was applied from fixed-wing aircraft to approximately 700 acres of western spruce budworm host type at the rate of 12 billion international units (BIU's) per gallon of water per acre. The objective of this control project was foliage protection to insure future cone production for this timber sale area. Evaluations, 20 days after spray application, showed the adjusted defoliation was 38.2 percent in the sprayed areas and 66.4 percent in the unsprayed areas at the termination of the budworm feeding period. Population reduction, corrected for natural mortality at 20 days, was 68 and 81 percent at the time of moth emergence.