When the larvae of Pseudaletica unipuncta were reared at 37°C after exposure to a nuclear-polyhedrosis virus, either by per os or by intrahemocoelic inoculation, they did not succumb to virus infection. Histopathologically, however, a small number of cells of the fat body, hypodermis and trachea were infected with virus and contained virus particles but no polyhedra. Autoradiographic results revealed that viral DNA synthesis occurred in a few cells of virus-exposed larvae held at 37°C, but the polyhedron-protein synthesis did not occur.These results indicated that high temperature (37°C) may prevent the lethal infection of virus-exposed larvae not only by reducing the mechanism of adsorption and/or penetration of the virus into the cell, but also suppressing the development of polyhedra.