Recent work has shown that Fusarium circinatum, previously known only as a pathogen of pines (Pinus) and Douglas fir (Picea abies), can also infect species in the grass family Poaceae. The present study characterized the ability of F. circinatum to colonize corn (Zea mays), in comparison to closely related Fusarium species known to have endophytic associations with Z. mays. When inoculum was applied to seed, F. circinatum colonized roots, stems and developing ears of Z. mays, roots were colonized on plants grown in infested soil, and corn ears became infected when inoculum was applied to husk wounds or through silk channels without wounding. Colonization was restricted to intercellular growth in the cortical and epidermal tissue. No negative effects on growth or emergence were detected. These results constitute the first description of an endophytic association of F. circinatum with a grass species and are illustrative of ecological activities that may remain undetected where fungi have no visible impact on their plant host.