Traditionally, Cordyceps species have been used as a part of herbal medicine in Oriental countries, including Korea for internal health, vigor and to cure different diseases related to heart, lung etc. In recent years, research on artificial fruiting of some species of the genus Cordyceps including C. militaris has been carried out extensively because of their medicinal value. Instability observed in the in vitro fruiting of C. militaris is reported in the present study.