Penetration of Monosporascus cannonballus into and growth within cantaloupe roots was studied using light and electron microcopy. Germ tubes penetrated the epidermis, and hyphae grew, without branching, almost directly to the xylem. The hyphae traversed the endodermis into protoxylem cells, and then grew extensively within the lumen of metaxylem vessels. Eventually, the hyphae grew back out into the cortical cells. A relatively low percentage of cells within both the cortex and xylem of lesions contained hyphae. The hyphae were generally localized within the lesion and could rarely be isolated more than 2 mm away from the margin of the lesion. Regardless of tissue type, hyphae were predominately intracellular. M. cannonballus appeared to be most similar to vascular wilt pathogens in its mode of parasitism, but does not spread via the vascular system to above-ground plant tissues.