Gypsy moth females that survived inoculation with Lymantria dispar (L.) nucleopolyhedrovirus (LdNPV) as fifth instars were smaller as pupae and laid fewer eggs as adults. Treatment with both wild type virus containing the egt gene and a genetically manipulated, egt- virus, lacking this gene, reduced pupal mass to a similar degree. Sublethal infection with wild type virus reduced the masses of surviving pupae at 20, 25, and 28°C. A relationship between virus dose (5,000, 50,000 and 500,000 occlusion bodies per larva), mass of pupae, and egg mass size only occurred in one experiment in which larvae were reared at 25°C and inoculated 5 d after molt to the fifth instar. Vertical transmission of overt infection occurred in two of 13 egg masses (15%) produced by females inoculated with virus as larvae. The five larvae infected with virus were ≈0.5% of the larvae tested. Whether sublethal effects of LdNPV infection occur in field populations of gypsy moth remains to be tested.