Larvae of the poplar defoliator, Clostera cupreata Butler (Lepidoptera : Notodontidae) were recorded dying due to the infection of a fungus identified as Aspergillus flavus Link. This is the first record of A. flavus infecting the larvae of C. cupreata. Pathogenecity of the fungus was confirmed by developing its culture and infecting the fresh larvae in the laboratory. Application of the fungus is discussed in the light of its carcinogenic mycotoxin - Aflatoxin B1.