Aerial application of mexacarbate and Bacillus thuringiensis Berliner was tested against pine butterfly, Neophasia menapia (F. and F.). These were applied to 40-acre plots on the Bitterroot National Forest and adjacent State and private lands during June 1973. Two concentrations of each material were tested; these were 0.15 and 0.30 pound per acre of mexacarbate, and 0.5 and 1.0 pound of B. thuringiensis per acre. Both concentrations of mexacarbate caused pine butterfly population reductions of near 97 percent and B. thuringiensis provided 73 percent reduction at the • 0.5 pound rate and 92 percent at the 1.0 pound rate. Natural decline of late instar larval population precluded an evaluation of differences in degree of defoliation between treated and untreated areas.