Cryphonectria parasitica isolates infected by viruses of the genus Hypovirus become hypovirulent and reduce the severity of the chestnut blight disease. In order to detect the presence of hypovirulent strains in Castilla y Leon, 117 isolates were studied. Isolates were first analyzed morphologically looking for abnormal pigmentation and reduced production of asexual sporulation in culture. After that, all 117 isolates were screened for the presence of dsRNA, the genome of the hypovirus. Eight isolates of vegetative compatibility type EU1 and four of EU11 were found to contain hypoviral dsRNA. All the hypovirulent isolates were from the Leon province where chestnut blight is affecting chestnut trees since 1978. In contrast, all the isolates tested from Zamora, Avila and Salamanca, where chestnut blight has appeared recently, were hypovirus-free.