In August 1965, 135,000 acres of jack pine forest in the St. Maurice Valley, Quebec, were sprayed with Phosphamidon for control of an outbreak of the Swaine jack pine sawfly, Neodiprion swainei Middleton. Applied by spray planes at a rate of 0.2 gallon per acre containing ¼ pounds of active insecticide material, it resulted in 99% mortality of the sawfly, and the threat of defoliation subsided. Phosphamidon applied at [Formula: see text] lb. per acre in a subsidiary test was almost as effective. One year after treatment, the sawfly population remained suppressed in the spray area, with no immediate prospect of resurgence. The treatment did not adversely affect the balance between the sawfly and its parasites, nor cause long-term instability in populations of other insects, birds and mammals living in the forest.