Higher mortality from nuclear polyhedrosis virus (NPV) was observed among gypsy moth larvae hatched from egg masses laid on virus-treated bark substrates than larvae hatched from egg masses laid on bleach-treated or untreated bark. Eggs from the inner portions of the egg mass (proximal to the substrate) were more heavily contaminated than those in the outer portions. These results indicate that gypsy moth egg masses can become contaminated with infectious NPV during oviposition on contaminated substrates. This mechanism may be an important mode of transmission in high-density populations or in years following an epizootic.