Vertebrate predators killed about 70% of the gypsy moth,Lymantria dispar(L.), pupae in a series of sparse, numerically stable populations studied in New England between 1965 and 1968; and white-footed mice,Peromyscus leucopusRaf., were the most important predators. Pupae in the litter were more likely to be preyed upon than those in other locations, and female pupae were more likely than males to be killed by vertebrates. All other agents (primarily parasites) killed about 15% of the pupae. It is postulated that the growing insects exhibit behavior that enables them to evade natural enemies in Europe, by resting and pupating in the litter at the bases of their host trees. In North America, however, this behavioral trait often assures not only high gypsy moth mortality but also the destruction of many of the parasites that may have caused this trait to evolve. It is also postulated that a new behavioral adaptation may be evolving by this pest in response to new conditions in North America. Certain management implications from the above findings and postulates are described, and further research studies are recommended.