Cadavers of late instars of the gypsy moth,Lymantria dispar(L.), that were attached to tree boles were examined to determine cause of death. Cadavers of gypsy moth larvae killed by the fungal pathogenEntomophaga maimaigaHumber, Shimazu&Soper were predominantly oriented vertically with heads downward, all prolegs were frequently at a 900 angle to the axis of the body, and older cadavers were usually dry in appearance. By contrast, larvae killed by the L.disparnuclear polyhedrosis virus (NPV) were predominantly positioned with anterior prolegs attached to the bole, the anterior section of the body hanging unattached, and the body bending at an angle of<90°. Cadavers of NPV-killed larvae remained soft and moist when older, and all prolegs were seldom at a 90° angle to the body. In 4.4% of cadavers analyzed, mixed infections with both fungus and virus were evident. Individual samplers with little field experience with these pathogens averaged correct field diagnosis of 69.9% of cadavers of pathogen-killed larvae in the field using these criteria. Cadavers of larvae that had died only recently were more frequently incorrectly diagnosed. It is concluded that field diagnosis of the cause of death of gypsy moth larvae due to E.maimaigaor NPV leads to an unacceptably high level of error for quantitative ecological studies. Nevertheless, cadaver signs and symptoms can provide generalized information on the occurrence of these pathogens.