The parasitoid, Peristenus howardi Shaw (Hymenoptera: Braconidae) has been found to parasitize a large proportion of Lygus species in some Washington and Idaho alfalfa seed fields. During 2002-2003 a survey was conducted to estimate the proportion of Lygus spp. (Hemiptera: Miridae) parasitized and the amountof that parasitism attributable to P. howardi in crop and non-crop plants attacked by Lygus in the alfalfa seed growing region of southwestern Idaho and eastern Oregon. Percentage parasitism was estimated from dissection of field-collected Lygus nymphs. Polymerase chain reaction (PCR) was used to amplify DNA extracted from parasitoid larvae followed by restriction endonuclease digestion of PCR products to distinguish P. howardi from other potentially co-occurring Peristenus species. Peak parasitism of Lygus nymphs occurred between the first and third weeks of July for both years for all host plants sampled. Of the parasitoid larvae recovered from Lygus nymphs in our study, 75% to 80% tested positive as Peristenus spp. and 76% of these matched the endonuclease digestion banding pattern for P. howardi. The identity of the remaining 20% to 25% of the parasitoids is not known.