The quantities of vitellogenin present in large and small gypsy moth [Lepidoptera: Lymantriidae: Lymantria dispar (L.)] eggs were determined serologically. Larvae developing in large eggs have significantly greater yolk resources available during embryological development, and also retain a larger amount of yolk reserve for post-diapause consumption. High temperatures during embryonation and immediately preceding eclosion caused depletion of yolk. High temperatures did not induce depletion during the diapause period. The relationship of yolk to population quality and the possible consequences of yolk depletion are discussed.