Within 1-ha plots, gypsy moth,Lymantria disparL., densities were artificially elevated from low densities (about 20 egg masses/ha) to high densities (8,000 egg masses/ ha) using field-collected eggs and F1sterile laboratory-reared eggs. At all sites, including the feral and sterile release areas, densities decreased to undetectable levels (less than 10 egg masses/ha). In the sterile and feral release areas, parasitism byCotesia melanoscela(Ratzeburg) was the largest source of identified mortality and was significantly greater than in the surrounding area or in untreated plots. Within the feral release area, parasitism byCompsilura concinnata(Meigen) andParasetigena silvestris(Robineau-Desvoidy) was substantial and was greater than in the surrounding area. This pattern was not observed in the sterile release area. This difference may have been caused by differences in gypsy moth phenologies between the two populations. Spatially density-dependent mortality caused by parasitoids is hypothesized to be important in the regulation of low-density populations. Furthermore, this mortality may have profound effects on attempts to suppress populations using releases of induced- inherited sterile insects.