HatchingPorthetria dispar(L.) larvae acquire nuclear polyhedrosis virus from the egg surface or from contaminated hair as they emerge from the egg mass. In comparative tests hair appeared to be more infectious than eggs. The most heavily contaminated material was debris taken from sites used for larval shelter and pupation. One hundred percent mortality occurred in hatching larvae exposed to the composite of exuviae, larval and pupal cadavers, and empty pupal cases. Debris mats are characteristic of dense but not sparse gypsy moth populations.