A double application of Disparvirus, a nuclear polyhidrosis virus, at 1.25 × 1012 polyhedral inclusion bodies (PIB) per hectare, giving a total of 2.5 × 1012 PIB per hectare, was applied aerially on three plots in an emitted volume of 10.0 L per hectare. The two applications were 3 days apart and most larvae were in the first instar. Three plots were selected as untreated checks; each was paired with a treated plot on the basis of pre-spray gypsy moth egg-mass numbers and locality. The pre-spray egg-mass counts ranged from 1430 to 8520 per hectare in the six plots. Assessment of the treatment was based on numbers of pupae and fall egg masses as well as on estimates of defoliation and on microscopic examination of larvae collected at weekly intervals to determine the incidence of virus infection. Between 12 and 19 days post-spray, 49, 61, and 85% of the larvae were infected with virus in the three treated plots compared with 3, 7, and 14%, respectively, of larvae in the check plots. Red oak was 14% defoliated in two of the treated plots compared with 82 and 90% in their corresponding check plots. The third plot suffered 46% defoliation due to leaf-eating caterpillars other than gypsy moth; defoliation in its corresponding check plot was 31%. Reductions in egg-mass numbers in the treated plots were 76, 93, and 98% compared with an increase of 56% and decreases of 50 and 70%, respectively, in corresponding check plots. Corrected population reductions (Abbott’s formula) were 84, 85, and 92% in the three treated plots.