The influence of defoliation on gypsy moth fecundity and male moth wing length and the relationship between male moth wing length and egg mass density were investigated. Male pupae and egg masses were collected from plots located in the leading edge area of the gypsy moth infestation in Virginia and Maryland. Defoliation and egg mass density estimates were made in each plot. Male pupae were allowed to emerge, and the left forewings of 2,636 moths were measured to determine wing length. Fecundity was determined volumetrically for 767 egg masses. Gypsy moth fecundity and male wing length decreased substantially only when defoliation exceeded a threshold level of ≍40%. There was a minimal change in fecundity and male wing length due to increasing levels of defoliation above or below this threshold. Thus, the effect of defoliation on male wing length and fecundity is essentially binomial (i.e., male wing lengthy is either large or small and fecundity is either high or low). Wing length of male moths explained only ≍10% of the variation in egg mass density in the area where pupae were collected. The wing length of moths captured in pheromone traps used in conjunction with the number of moths captured may be useful for monitoring gypsy moth populations.