Simultaneous infection of CIV and nuclear polyhedrosis virus (NPV) of the silkworm, Bombyx mori occurred in both Galleria mellonella larvae and B. mori pupae. LD_50-time curve of NPV in simultaneously infected silkworm pupae with both viruses was very similar to that in pupae infected with NPV alone. It was also found that there was no difference between multiplication of CIV in simultaneous infection and that in single infection of CIV. In simultaneously injected Galleria larvae with CIV and NPV, hemocytes formed concurrently nuclear polyhedra in nuclei and cytoplasmic inclusions of CIV were sometimes observed. From these results, it was suggested that there were no synergistic effect or interference between CIV and NPV of the silkworm.