SUMMARYOne hundred and eighty-three species of mycophilic fungi have been isolated from nature and studied for their fundamental parasitic properties. Examination was in paired culture with the host both in vitro and in vivo. The suggestion is made that mycophilic fungi be divided into six physiological groups: biotrophics, facultative necrotrophics, facultative biotrophics, necrotrophics, semisaprophytic mycophilics, and saprophytic associates.