Biological control researchers generally think of insect predators as preying exclusively on herbivorous arthropods. Thus, investigations of predator-predator interactions have focused on aspects of competition for prey, and most studies have concluded that predators influence the structure of insect communities by suppressing herbivore populations. Here we suggest that this perspective may need to be modified. We manipulated the community of generalist predators associated with the cotton aphid, Aphis gossypii Glover, to address two questions: (1) Do generalist predators consume other predators as well as herbivores? and (2) If predators do consume other predators, is the interaction strong enough to disrupt the control of herbivore populations? Direct behavioral observations in the field revealed that several predators do consume other predatory insects with which they may also compete for prey (intraguild predation). Field enclosure/exclosure experiments in which we manipulated the predator community on individual plants revealed that predation was a major source of mortality for larvae of the green lacewing, Chrysoperla carnea (Stephens). In one experiment this resulted in the disruption of highly effective biological control exerted by lacewing larvae. We conclude that generalist predators frequently prey on other predators, and that these interactions may impose important limitations on the efficacy of biological control.