This article reports the results of a long-term study conducted in province Manisa, Ege region, on biological control of chestnut blight (caused by Cryphonectria parasitica) by using hypovirulent strains and on their natural spread in the stand between the years 2002- 2009. Since hypovirulent strains were not found in the chestnut orchards of the region, hypovirulent isolates from the Marmara region which belong to vc type EU-1 and MAT-1 mating type were used in the study. The virulent strains isolated from the infected trees in the research area have revealed that they also belong to vc type EU-1 and MAT-1 mating type. After determination of the canker dimensions and types on the young chestnut trees, the introduced hypovirulent strain was inoculated into the holes around these cankers and the dimensional changes of the diseased areas in these trees were measured at monthly intervals and recorded. It was observed that most of the cankers treated with the hypovirulent strain on chestnut trees were healed between 2004 and 2005. The hypovirulent strain could be re-isolated from the healing cankers. Five years after inoculation, 45 new healed cankers were determined on 31 trees in the research area between 2007 and 2009, showing that hypovirulence disseminates naturally in the chestnut stand.