Parasitism of cornstalk borers was evaluated near Rio Bravo, Tamaulipas, Mexico, from 1984 to 1988. Additional surveyswere made on grain sorghum and johnsongrass. A total of 12,897 larvae and pupae ofDiatraea lineolata(Walker),D. saccharalis(F.), andEoreuma loftini(Dyar) was collected from 10,820 host plants. Total parasitism throughout this study was 6.8%, with no small larval or pupal parasites detected in corn. Six indigenous parasites were reared from all borer species and host plants.Chelonus sonorensisCameron attacked onlyE. loftiniand was the most common native parasite with 18% overall parasitism. Percentage of parasitism byChelonus sonorensiswas highest (34%) during the fall seasons, particularly at the vegetative corn stage. The remaining indigenous parasites appeared sporadically at very low levels of parasitism and includedDigonogastrasp.,Orgilus gelechiaevarus(Cushman),Glyptapantelessp.,Spilochalcissp., andEucelatoriasp. The exotic parasitesCotesia fiavipes(Cameron) andAllorhogas pyralophagusMarsh were field released during the fall of 1985.A. pyralophaguswas not recovered from field collections.Cotesia fiavipeswas recovered fromD. saccharalisandD. lineolataimmediately after releasesbegan and became the most abundant parasite of those borer species thereafter.Cotesia fiaoipeswas recovered most frequently during the fall during the reproductive and senescent corn stages. Average parasitism byCotesia flavipesduring the fall was 13% onD. saccharalisand 5% onD. lineolatawith the highest parasitism (53% onD. saccharalis) during the fall of 1987. The consistent recoveries ofCotesia fiavipesfrom 1985 to 1988 suggest that this parasite is permanently established in northern Tamaulipas, Mexico