Chestnut blight is controlled in Europe by using Cryphonectria hypovirus CHV1, a non-encapsulated RNA virus. The chestnut blight fungus, Cryphonectria parasitica, is weakened by the virus, and healing tissue growth occurs in the host tree. Transmission of this cytoplasmic hypovirus is restricted by the incompatibility system of the fungus, so that the hypovirus can be transmitted only between isolates of the same or closely related vegetative compatibility (vc) types. Hypovirulent isolates of C. parasitica (all of the French subtype CHV1-F1) from Castilla y León (NW Spain) were compared with virulent isolates in both laboratory (cut stems) and field inoculations (in two orchards in the province of León and one orchard in the province of Zamora). The tests were performed with the most common vc types in the region, EU1 and EU11. The cut stem assay revealed that the hypovirulent isolates of vc type EU1 did not reduce the growth of virulent cankers. By contrast, four hypovirulent strains H1, H4, H5 and H6 (all vc type EU11) reduced the growth of virulent isolates in the cut stem assay. Field tests showed that hypovirulent isolates of EU1 and EU11 were effective in reducing canker in both orchards in León with all treatments tested; however, in Zamora, where only EU11 was tested, all the treatments failed except H1, which was able to reduce growth of the canker eighteen months after the inoculation. The development of hypovirulence suggests that hypovirus subtype F1 is well adapted in the province of León. Both naturally extended and inoculated hypoviruses appear to have reduced the incidence of the canker, thus improving chestnut stands. However, the inoculations were not as effective in the orchards in Zamora. This indicates that the disease could be controlled in Castilla y León by inoculation of trees with hypovirulent strains, but that more tests should be done in provinces where the hypovirus is still not present.