A high proportion of the potato seedlings from the progenies of crosses between X tolerant parents, Katahdin being common to each cross, gave'light to medium necrotic reactions on leaves inoculated with a virus X complex. Localization of virus X was incomplete, and a small percentage of the necrotic reactors acted as strain separators so that masked, mild, severe, and necrotic strains of X were obtained. Strain separation by potato seedlings was influenced by environmental conditions.