The ubiquitous entomopathogenic fungus Beauveria bassiana (Balsamo) Vuillemin (Deuteromycotina: Hyphomycetes) has been extensively investigated to suppress populations of several insect species worldwide. In studies investigating the response of inbred maize lines to plant pathogenic fungi, B. bassiana was isolated from the pith of plants. Subsequent field studies have shown that B. bassiana placed in or on maize plants formed an endophytic relationship, persisted for greater than 90 days and remained virulent to insects throughout the growing season. Even though these field studies demonstrated the potential insecticidal use of this relationship, the manner in which B. bassiana colonizes Zea mays L. was not known;The following study was initiated to elucidate the methods of adhesion, penetration and colonization of maize by B. bassiana. Light and electron microscopic techniques were utilized to investigate the interactions between these two species. Understanding this unique relationship will be invaluable in further development and utilization of this fungus to manage insect pests of food plants;The adhesion and subsequent growth and penetration on maize leaves by B. bassiana is very similar to the germination and penetration by B. bassiana on insect cuticles. The hydrophobic conidia attach to the leaf surface, germ tubes form, and hyphal elongation occurs along the leaf surface. The hyphae appear to penetrate directly the maize leaf cuticle and enter the airspace and/or mesophyll cells. Apparently B. bassiana does not require specific topographic signals at an appropriate entry site as do some phytopathogenic fungi;Long hyphal structures were observed to follow the leaf apoplast in any direction from the point of penetration. Hyphae were observed to penetrate the vascular tissue. Because vascular bundles are interconnected throughout the maize plant, this may explain how the fungus travels within the plant and ultimately provides overall insecticidal protection. The fungus did not lose any virulence toward the European corn borer, Ostinia nubilalis (Hubner) after colonizing maize;This is the first report illustrating the mode of penetration by B. bassiana into Z. mays. This special relationship between an entomopathogenic fungus and a plant suggests possibilities for biological control, including utilization of indigenous fungal inocula as "insecticides."