This is principally a review article, summarizing the present information on the parasitism and nutrition of fungi parasitic on other fungi, with emphasis on recent work done at West Virginia University. A theory is proposed to explain the host-parasite relationship involving 2 biotrophic mycoparasites (Calcarisporium parasiticum and Gonatobotrys simplex) and their hosts. Both are deficient for an undeterminedgrowth, factor, which must be absorbed from the living host cell or from the substrate to which this nutrient is added. The ability of these myco-parasltes to absorb the required nutrient from living host cells, probably by altering the permeability of the cell membrane, appears to be the basis of successful parasitism.