Technical Abstract: In addition to damaging trees, the eastern tent caterpillar, (Malacosoma americanum (F.)) is implicated in early fetal loss and late-term abortion in horses. In a field study, we evaluated the potential biological control of eastern tent caterpillar using eastern tent caterpillar nuclear polyhedrosis virus (ETNPV), a naturally occurring virus that is nearly species specific. Egg masses were collected and hatched in the laboratory then second instar larvae were inoculated and the virus propagated in vivo. The following year, a viral pesticide was formulated using 72% distilled water, 25% molasses, 1% folic acid, 2% Rhoplex B-60 A® as a spreader-sticker, and a measured quantity of viral polyhedra. Virus concentrations of 104, 106 and 108 polyhedral inclusion bodies (PIB)/ml were applied to foliage on which second, third and fourth instar caterpillars were feeding. The larval colonies were collected and the surviving caterpillars counted. Treatment means were compared via 95% bootstrap confidence intervals. The data indicate fourth instar caterpillars are fairly resistant to the virus. Alternatively, third instar caterpillars are somewhat susceptible to the virus and second instar caterpillars are highly susceptible, but only at the highest concentration tested. Our data indicate ETNPV can be propagated, harvested and refined for formulation as a biological control of eastern tent caterpillar. Its use on this insect may be merited in circumstances where landowners and managers want to deter outbreaks and maintain low endemic population levels.