SUMMARYBacteria are frequently associated with freshly ejected ascospores ofHypoxylon pruinatum, and are capable of reducing or preventing ascospore germination. In Minnesota 72% of the cankers had perithecia contaminated with bacteria and bacteria increased when stromata were kept moist continuously. When bacteria were suppressed spore germination was increased as much as 94%. Bacteria isolated from healthy bark also inhibited ascospore germination and may prevent establishment ofHypoxylon pruinatumin aspen.