In the summer of 1958 a virus disease of larvae of Neodiprion swainei Midd., a serious defoliator of jack pine (Pinus banksiana Lamb.), was investigated. The experiments were carried out near St. David de Falardeau in the Lake St. John region, Quebec, where the forest is largely composed of 15- to 20-year-old jack pine. The infestation was three years old, and judging by the production of eggs, had reached serious proportions in some places.