Cooperative Forestry and Pest Management conducted a pilot control project in central Montana to determine the effectiveness of two commercial formulations of Bacillus thuringiensis Berliner applied at 8 billion international units (BIU) per acre (19.8 BIU per hectare) when used under operational conditions against the western spruce budworm, Choristoneura occidentalis Freeman. Nine blocks 2,000-4,200 acres (810-1,701 hectares) were selected in areas of budworm infestation on State, private, BLM, and Forest Service lands near Butte, Montana. Three blocks were randomly assigned Dipel 4L treatment, three blocks were assigned Thuricide 16B treatment, and the remaining three served as control blocks. Time and sequence of treatment was determined by larval development sampling in each block. Blocks were released for treatment when 50 percent of the larvae had developed to or beyond the fourth instar. A prespray population sample was collected within 24 hours of spraying from 25 three-tree cluster plots established in each block. Treating began on June 29 and ended on July 5. Spray was applied with two Ayers Turbo Thrush aircraft equipped with fulljet one-eighth 66A8W nozzles calibrated at 1 gallon per acre (1.5 liters per hectare). Dipel was applied to 8,600 acres (3,493 hectares), and Thuricide to 8,150 acres (3,301 hectares). Eight thousand acres (3,240 hectares) were used as control areas. Postspray population samples were collected at 7, 14, and 21 days following treatment. Population densities are expressed in number of larvae per 100 shoots. Following egg deposition, percent defoliation and egg mass densities were collected from each cluster tree. Unadjusted budworm mortality at 21 days was 48 and 62 percent for Dipel and Thuricide respectively. Budworm mortality in control blocks averaged 26 percent. Percent defoliation was 33, 36, and 47 for Dipel, Thuricide, and control. Adjusted mortality for all larvae (combined) prior to the egg stage was 73 and 91 percent for Dipel and Thuricide. In 1981, percent parasitism for all defoliators in the Dipel, Thuricide, and control blocks was 21.5, 19.8, and 26.0, respectively. Egg mass densities per square meter (per square foot) in 1981 were 18.3 (5.6), 17.6 (5.4), and 38.5 (11.7) for Dipel, Thuricide, and control. Mean budworm larvael densities in 1982 were 4.4, 3.5, and 12.7 larvae per 100 shoots for the Dipel, Thuricide, and control areas. Rearing samples showed a greater proportion of females emerging in sprayed than in control blocks (61.4 and 62.0 percent for Dipel and Thuricide compared to 52.4 percent in the controls); however, adult female weight was higher in the control blocks (126.3 mg compared to 121.3 and 113.9 mg for the Dipel and Thuricide blocks, respectively). Average percent parasitism of budworm larvae and pupae in 1982 was 5.1, 8.6, and 7.3 for the Dipel, Thuricide, and control blocks. We were unable to distinguish any effects of B.t. on individual budworm parasite species during the study. Egg mass densities in 1982 were 44.5 (13.6), 39.7 (12.1), and 78.6 (24.0) per square meter (per square foot) for Dipel, Thuricide, and control.