Field experiments were conducted to examine interactions of Heliothis virescens (F.), its natural enemies, and tobacco plants engineered to express low levels of Bacillus thuringiensis Berliner toxin. Survival of H. virescens larvae was measured in response to four treatments: (1) toxin-producing plants exposed to natural enemies, (2) toxin-producing plants caged to exclude enemies, (3) toxin-free plants exposed to enemies, and (4) toxin-free plants caged to exclude enemies. B. thuringiensis toxin mediated resistance caused a significant decrease in first-instar survival, and natural enemies caused a significant decrease in third-instar survival. Survival of uncaged first instars as a proportion of survival of caged first instars was significantly lower on toxic plants than control plants, indicating synergism of resistance and natural enemies. Among collections of artificially infested larvae, parasitism by Campoletis sonorensis (Cameron) was significantly higher on toxic plants than on control plants, another indication of synergism. Among collections of wild larvae and larvae censused in the field, parasitism usually did not differ between plant lines. Larval development of H. virescens was significantly slower on toxic plants than on control plants in two out of five trials. Prolonged vulnerability to natural enemies appeared to provide a mechanism for synergism. However, synergistic increases in mortality and parasitism were detected in two trials when development rates on toxic plants and control plants were equal, indicating existence of another mechanism. B. thuringiensis toxin-mediated partial resistance appeared compatible with natural enemies for control of H. virescens . However, a simulation using a theoretical population genetic model suggested that synergism of the level measured in this study could accelerate pest adaptation to resistant plants.