Stands of American chestnut trees have survived in Michigan in spite of the presence of Endothia parasitica. Cankered trees were evaluated in eight groves in seven of which two types of abnormal cankers were observed. These abnormal cankers resulted when diseased periderm was replaced by wound periderm. Endothia parasitica isolated from abnormal cankers often differed from E. parasitica isolated from normal cankers in that the isolates had abnormal culture morphology, reduced virulence, and contained double-stranded ribonucleic acid (dsRNA). These characteristics are similar to those of hypovirulent strains of E. parasitica found in Europe, except that four different dsRNA banding patterns were found in Michigan isolates, indicating different dsRNA genomes. Pathogenicity tests showed that host resistance or tolerance probably is not responsible for the prolonged life of infected trees in certain groves. Therefore, native Michigan hypovirulent isolates of E. parasitica may be responsible for the recovering American chestnut groves in Michigan.