Intensive sampling of the pathogenic fungus Heterobasidion annosum in white fir stands yielded a large percentage of homokaryotic isolates. To assess the stability and virulence of homokaryotic isolates, four homokaryons and four heterokaryons were inoculated on 200 roots from 50 mature trees. Roots were sampled after 4 and 12 months. Eighty-three percent of the inoculated roots were infected, and 95% of the isolates retrieved were identified as the isolates originally inoculated. Homokaryons were not heterokaryotized and no significant colonization difference was observed between homokaryons and heterokaryons. One year after the removal of inoculated roots and the subsequent creation of large basal wounds, fungal isolates that had reached the root collar and colonized the main stem of the tree could still be isolated. Eight putative heterokaryons and six putative homokaryons were obtained from stems of a subsample of the inoculated trees and were grown in the laboratory. Six of eight heterokaryons remained unchanged, whereas all six homokaryons had been heterokaryotized.