About half of the living yellow birch (BETULA LUTEA Michx. f.) in New Brunswick and Nova Scotia contain dead limbs as a result of dieback. At present approximately one-third of these dead limbs contain decay. Thus far, the fungi isolated from these decays appear to be saprophytes. FOMES FOMENTARIUS (L. ex Fr.) Kickx has been isolated more frequently than any other of the fungi found in the limbs. The probability and possible consequence of certain of these fungi having the capacity of advancing downward in living trees is discussed.