Technical Abstract: Fusarium moniliforme is a nonobligate parasite of corn, which exists as a complex of closely related fungi from different mating populations or biological species. Strains of the fungus isolated from corn have been determined to belong to mating populations A, although other populations have been isolated from corn. The ultrastuctural association of the fungus swith corn during growth and the effects of the host on suppression of disease are reviewed. This fungus enters a relationship with corn cultivars that is not always pathogenic. Pathogenesis is delayed, if it even occurs. F. moniliforme can exist entirely as an endophyte, systemically colonizing kernels, remaining there until germination upon which the fungus infects the emerging seedlings. The symptonless association persists during the growth cycle of corn, and the resulting endophytic hyphae may be the source of mycotoxin production. The host's ability to suppress the fungus appears to be related to one class of compounds, the cyclic hydroxamic acids and their decomposition products, which can be catabolized by the fungi of mating population A but not C.