85 isolates of soil fungi, part of which may be characterized as mycorrhiza fungi, were included in a screening test against the root rot fungus Fomes annosus. It was arrested by 40% of the number and an unoccupied, bright and measurable zone then divided the two microorganisms. Some different types of influence were observed and interpreted. A wide antagonistic spectrum was demonstrated both within and between the species. One isolate of the mycorrhiza fungus Boletus bovinus, one of B. variegatus and one from roots of Monotropa hypopitys inhibited F, annosus most strongly. The inhibition established was of a fungistatic type. In order to widen the knowledge of the variation in sensitivity of F. annosus, 85 isolates mainly from field trials in Norway spruce were selected and screened to three strongly active mycorrhiza-forming fungi. As a rule all root rot isolates were highly sensitive to all three antagonists. Only three exceptions mere obtained. The same kind of pronounced sensibility against toxins produced was demonstrated, when F. annosus was grown in nutrient solutions precultivated with some active mycorrhiza fungi. Finally the importance of inoculation time on the antagonism was studied on solid media. A strong correlation exists between the time advantage and the toxic level obtained