The red sunflower seed weevil, Smicronyx fulvus LeConte, is a pest of sunflower in the northern and central Plains sunflower production regions. Weevil larvae feed and develop in the sunflower achene dropping to the soil to overwinter. A total of 630 parasitoids emerging from seed weevil larvae recovered from North and South Dakota and Minnesota from 1991 to 1995 was identified as Triaspis aequoris Martin, a solitary koinobiont endoparasitoid. The mean parasitization rate for the red sunflower seed weevil by T. aequoris ranged from 2.5 to 24.2% per year. There was an increase in the percentage parasitization as overall weevil populations decreased over years. In Nebraska, percent parasitization by T. aequoris, the only species recovered, increased with increasing densities of S. fulvus between 1993 and 1995. Patterns of emergence for both red sunflower seed weevil and T. aequoris were similar in two locations in North Dakota. Oviposition pattern in seeds in the sunflower head showed decreasing density toward the center, but T. aequoris parasitization was equal throughout the head. Triaspis aequoris, an egg-larval parasitoid, effectively searched for and attacked weevil eggs as soon as weevil oviposition had begun in the field. Date of planting studies showed that damage from weevils increased as seeding of fields was delayed, but parasitization of weevil larvae was similar among all dates. Activity by T. aequoris may have contributed to the decline of the red sunflower seed weevil from North and South Dakota and Minnesota. Lower densities of weevils also may account for the reduction in the diversity of parasitoid species of this weevil. The parasitoid appears to be well adapted to its host, efficiently parasitizes the red sunflower seed weevil and is amenable for use with some pest management strategies in cultivated sunflower.