Gypsy moth defoliation of oak trees has been shown to lead to increased tannin levels, which, in turn, lead to reduced gypsy moth growth and fecundity. In laboratory experiments, increased tannin levels can interfere with the transmission of a virus that is consumed by larvae on oak foliage, and high mortality rates of larvae in the field are sometimes associated with low levels of defoliation. These latter results have led to the suggestion that gypsy moth defoliation may cause reduced mortality attributable to the virus by elevating oak tannin levels. In a series of field experiments, we directly tested the hypothesis that gypsy moth defoliation of oaks leads to reduced virus transmission rates. In each of three study years, in oak forests with almost no naturally occurring gypsy moths or virus, we measured virus transmission rates in gypsy moths feeding on oaks, with and without experimental defoliation. By carefully synchronizing our experiments with the phenology of natural gypsy moth populations, we mimicked natural virus transmission processes during that part of the gypsy moth life cycle when virus transmission occurs. In our experiments, there was no effect of gypsy moth defoliation on tannin levels; consequently, virus transmission in both the field and the lab was unaffected by defoliation. Although we did observe increased tannin levels on more severely defoliated oak trees in one of two naturally defoliated oak stands late in the season, virus transmission had virtually ceased by that time. Our results suggest that gypsy moth defoliation does not affect tannin levels early enough in the larval season to have a measurable effect on the interaction between the gypsy moth and its nuclear polyhedrosis virus.