Lygus hesperus Knight and L. lineolaris (Palisot) are important pests of numerous fruit, vegetable, fiber, tree, and seed crops in North America. We discovered Peristenus howardi n. sp., described in this article, parasitizing L. hesperus nymphs in alfalfa grown seed in Idaho. Parasitism rates were surprisingly high, 81% in generation I (1998), and 44-51% in generation II (1997 and 1998). This braconid wasp may be a practical biological control factor for alfalfa and other seed crops, and further research is warranted. We found that P. howardi is a native species, is thelyotokous, has three generations per year and produces some diapausing parasites in each generation. The geographic range of this species is not yet known. P. howardi readily parasitized the eastern L. lineolaris in laboratory tests, so it is also a potential biocontrol agent for this related mirid species.