The nature of this parasitism is not essentially different from the parasitic relationship between microorganisms and higher plants. Certainly, some of the basic principles apply to both groups. The necrotrophic myco-parasites apparently destroy their hosts by means of enzymes or other nonspecific toxic substances and are then able to absorb nutrients from the killed host cells. They are characterized by wide host ranges and ability to grow on common laboratory media. Two biotrophic parasites are known to be deficient for a growth factor, which they must absorb directly from living host cells or from the substrate to which the growth factor has been added. While the absence of the growth factor would result in immunity, its presence in a fungus mycelium does not necessarily result in susceptibility. It is concluded that the ability of these 2 mycoparasites, Chytridium parasiticum and Gonatobotrys simplex, to absorb the required nutrient from the living host cells, probably by altering the permeability of the cell membrane, is the basic factor underlying their successful parasitism. There is evidence that the growth factor is a common intermediate metabolic product synthesized by and also required for growth of many fungi. Another type of biotrophic mycoparasitism is illustrated by species of Piptocephalis, which do not respond to the addition of the growth factor or other host extracts to the agar media. They are known to grow only in the presence of a living host. The degree of susceptibility of these hosts is correlated with low C-N ratio of the host medium and with high concentrations of soluble N in the host mycelium, but susceptibility has not been found to be related to any specific nutrient. A 3rd group of biotrophic mycoparasites is favored by a high C-N ratio of the host medium. This type is illustrated by Olpidiopsis incrassata and G. fuscum. There is some evidence that in the latter fungus parasitism is related to lipid synthesis by the host The answers to many questions relating to the nature of mycoparasitism must await the successful axenic growth of many of the biotrophic parasites and subsequent critical study of the nutritional requirements of each, together with studies of environmental and genetic factors. There is a bibliography with 30 references.