Abstract
Parasitism in natural populations of Hyphantria cunea Drury was measured in New Brunswick and Nova Scotia, 1957–1973. Thirteen parasite species were recovered during this period but only a few caused an appreciable degree of mortality. The per cent mortality showed an immediate rather than a delayed response to host density, the relationship being direct up to a density of about 10 host colonies per mile, then inverse. Possible reasons for this sort of relationship are discussed.