Chlorpyrifos is one of the most widely used insecticides worldwide. It has been shown to have deleterious effects on survival of nontarget insects, but its impact on behavior has received less attention. In this study, we investigated the sublethal effects of this insecticide on sexual pheromone discrimination in two Trichogramma species. In these species, sexual pheromones arrest partners belonging to the same species. This specificity is important for reproduction efficacy because interspecific matings are sterile. We used two sympatric and closely related species of Trichogramma to study how two doses (LD 20 and LD 0.1) of chlorpyrifos can alter the discrimination by males and the emission by females of pheromones from both species. When exposed to the LD 20, the males of both species showed a decrease in the discrimination of conspecific sexual pheromones. For one of the two species (T. evanescens Westwood), this decrease even led to a total annihilation of discrimination. A dose as low as the LD 0.1, inducing no apparent mortality, induced the same decrease in pheromone discrimination for T. semblidis Aurivillius males. However, no effect was observed on the discrimination by males of sexual pheromones emitted by females either exposed to an LD 20 or an LD 0.1 of chlorpyrifos. By decreasing the discrimination by males of sexual pheromones, chlorpyrifos may induce interspecific interactions and attempts at copulating that would decrease the fitness of parasitoids. The implications of these results in relation to environmental pollution, the mode of action of the insecticide and the status of natural enemy of Trichogramma are discussed.