It has been shown that saprophytes were prevalent at the nodes, lenticels, and other irregularities on 1-, 2-, and 3-year-old bark ofPopulus trichocarpaTorrey and Gray andSalixsp. Antagonism tests demonstrated that the extracts of natural bark and two of the most common saprophytes inhibited the growth ofHypoxylon pruinatum(Klotzsch) Cke. However, the extracts of surface-sterilized bark were found to promote the cultural growth of the pathogen. No differences were observed in the behavior of the extracts obtained from dormant and actively growing bark tissues.The results of inoculation experiments demonstrated thatHypoxyloncanker did not develop when the saprophytes were present on watered greenwood cuttings of black Cottonwood and willow. However, the saprophytes were not effective in preventing infection and canker growth on cuttings with below-normal levels in bark turgor. Surface-sterilized cuttings proved susceptible to the disease at all levels of bark turgor.Possible relationships between the temperature and moisture requirements of the pathogen and representative bark antagonists, and the epidemiology of canker diseases caused by facultative parasites, are discussed.