In the late summer of 1951 clearly visible mycelial mats of the oak wilt fungus were found in Illinois under the loose bark of wilt-killed trees. These mats were usually located beneath cracks in the bark; thus, they were exposed to the outside air and to visiting insects. Most wilt-killed trees contain beneath the bark numerous insect larvae of wood and bark boring beetles. Larvae were frequently found in direct contact with mycelial mats of the fungus. Larvae of the two-lined chestnut borer, _Agrilus bilineatus_, were most abundant, but larvae of species of the families Scolytidae and Cerambycidae were also present in large numbers. In addition to the mycelial mat under the bark there was often present a thick dark pad usually in the center of the mat. It is not known yet what part this pad plays in the life history of the fungus but we do know that it is produced by the same fungus which causes oak wilt. We also found in Illinois that the oak wilt fungus often develops into visible mats from chips of bark and wood that have been chopped from wilt-killed trees and allowed to lie on the moist forest floor. This should be remembered when considering sanitation as a partial means of controlling the disease. In 1951 the sexual or perfect form of the oak wilt fungus was produced on laboratory media in Missouri by crossing different strains of the fungus. The sexual form is recognized by the appearance of microscopic, black, short-beaked fruiting structures or perithecia that are filled with sticky ascospores. This sexual form is a species of _Endoconidiophora_. The sexual form of the fungus was first found in nature in Illinois in the autumn of 1951. The perithecia are produced on the mycelial mats beneath the loose and sometimes cracked bark of diseased oaks. Both the ascospores of the sexual form and the endospores or conidia of the asexual form will cause wilt if the spores are injected into oak trees. From the foregoing information it is apparent that several methods by which the disease might be spread over long distances are possible. First, and what seems to be most probable, is transmission by insects. Adult beetles, such as the two-lined chestnut borer, which emerge from dead trees in the spring and feed on the leaves of healthy trees might transmit the spores of the fungus. Other insects might feed on the fungus mats that are exposed through cracks in the bark and carry both the sticky ascospores and conidia to other trees. Additional agents that must be considered are woodpeckers, squirrels and air currents.