Three exotic larval parasites,Allorhogas pyralophagusMarsh andDigonogastra kimballiKirkland from Mexico andCotesia flavipes(Cameron) from the Indo-Australian region, were evaluated in enclosures in commercial corn fields in Castro County, Tex., against the southwestern corn borer,Diatraea grandiosellaDyar. Experiments conducted in 1986 measured the functional and numerical responses of the three parasites to varying host density. The effect of varying the number of searching parasites on parasitization and reproductive capacity was evaluated in 1987. The functional responses of the three parasites to host density were markedly different.D. kimballiexhibited the strongest response, which is hypothesized as a reflection of the coevolutionary history between this parasite and the southwestern corn borer. The responses of parasites to varying parasite density were not different among the three species. In both experiments, encapsulation of immature parasites ofC. flavipeswas observed. The numerical responses of the three parasites were weak; onlyC. flavipesproduced a greater number of female progeny than the number of females introduced per enclosure.