Parasitoid-pathogen interactions were examined using gypsy moth, Lymantria dispar (L.), Bacillus thuringiensis Berliner, and Compsilura concinnata (Meigen). The objectives of this study were to quantify effects of sublethal doses of Bacillus thuringiensis (Bt) force-fed to gypsy moths and to determine if sublethal doses of Bt affected host acceptance and suitability of gypsy moth for C. concinnata. Gypsy moths were minimally affected by sublethal doses of Bt; development of fourth instar was delayed, and male pupal mass reduced. Compsilura concinnata preferentially attacked and had higher superparasitism on noninfected hosts than on Bt-treated larvae. Exposure of gypsy moth to both sublethal doses of Bt and parasitoids reduced percentage parasitism and host larval survivorship. Effects on C. concinnata development varied with host superparasitism status. Parasitoids in Bt-treated, superparasitized gypsy moths had shorter larval development times and smaller pupal masses than parasitoids in untreated larvae, while parasitoids in singly parasitized larvae had larger pupal masses than those in superparasitized larvae. Timing of Bt infection relative to parasitism is a factor in gypsy moth mortality, but not in parasitoid potential fecundity.