an investigation was conducted o f the population structure, the somatic incompatibility reaction, and sporocarp overwintering in chondrostereum purpureum (pers.:fr.) pouzar, a proposed mycoherbicide of undesirable hardwood species. chondrostereum purpureum breeding populations o f recent cut-over sites in thunder bay, ontario were found to have a high number of individuals possessing different alleles for somatic incompatibility. up to nine fungal individuals per wood unit (logs, stumps) were found, with the majority of wood units having only one to three individuals. the macroscopic appearance of the somatic incompatibility interaction zone on malt extract agar varied among paired isolates and was occasionally ambiguous. phenotypic variability of the interaction zone ranged from scant mycelia to massed hyphae between the two colonies. microscopically the sparse interaction zone had chains of swollen spindle-shaped cells, while the massed interaction zone had distorted hyphae, encoded hyphae, and hyphal knots. of overwintered sporocarps collected in the spring o f 1998 and 1999, 86% produced viable spores. basidiospore levels in the spring may be greater than once thought, and thus may pose a threat to winter-damaged trees. this should be considered in the assessment of the epidemiology of c. purpureum, and in the assessment o f this fungus as a biocontrol agent.