The present investigation was prompted by observations of differences between trembling aspen clones in extent of hypoxylon cankering, and speculations about the possible role of bark microorganisms in preventing infection. The following hypothesis was advanced: aspen clones will differ in the composition of their bark and sapwood microfloras, and specifically, putatively resistant clones will have more kinds of microorganisms antagonistic to H. mammatum than putatively susceptible clones. Trembling aspen clones at the Pellston Plain research site were found to vary significantly in the composition of their bark and sapwood microfloras. Clones having a low percent of their ramets cankered by H. mammatum (resistant clones) also had more kinds of microorganisms antagonistic to the pathogen than clones having a high percent of cankered ramets (susceptible clones). ... In addition to differences in microorganisms, aspen clones were also found to vary in their response to isolates of H. mammatum. Clonal differences occurred in the size of lesions produced on excised aspen leaves tested with a culture filtrate of H. mammatum, and in the length of cankers resulting from inoculation of aspen branches with the pathogen. In general, resistant clones had small cankers and small lesions, and susceptible clones had large cankers and large lesions. ... Studies on the distribution of H. mammatum in aspen clones at the Pellston Plain research site have revealed that the fungus can occur non-pathogenically in the sapwood of aspen ramets in both susceptible and resistant clones. It has also been found that H. mammatum can develop in infection centers within aspen clones. These results suggest that perhaps the fungus is spreading in a systematic fashion within clones, and that the fungus might be present in aspen ramets well in advance of canker development. The methods by which this spread occurs are yet unknown, however, the possible role of a Lathridiid beetle as a vector of H. mammatum is being investigated. This beetle, Enicmus aterrimus, has been found regularly in association with hypoxylon cankers, and H. mammatum was routinely isolated from the exoskeleton, gut, and fecal material of larvae and adults.