A variety of factors were investigated to determine their influence on the epizootiology of the nuclear polyhedrosis virus (NPV) of the gypsy moth,Lymantria disparL. Egg masses laid on tree boles had substantially higher levels of contamination than those laid on rocks, ground debris, or understory brush. Egg masses laid on different tree species showed no differences except that those on black oaks were less contaminated at one of the three research sites. Egg mass contamination was not related to date of collection, compass orientation, or number of larvae hatched from the egg mass. Larvae collected from pitch pine had higher infection rates than those from either white or black oaks; however, the differences were not substantial. Mortality rates of larvae collected from different host species were even less distinctive. The implications for epizootiology and sampling are discussed.