Evidence indicates that the introduced parasitoidCollyria coxator(Villers) is providing adequate biological control ofCephus pygmaeus(L.) in Delaware.C. pygmaeusinfestations in wheat in 1982 and 1983 averaged ca. 2% while parasitism byC. coxatoraveraged 44% (range, 0–62%) in 1982 and 52% (range, 27–82%) in 1983. Postharvest (ca. July) sampling ofC. pygmaeuslarvae along field edges probably provides the best estimates of parasitism rates byC. coxator. There appears to be a strong density-independent parasitoid–host relationship within individual fields, regardless of field size. However, between fields, on an areawide basis, some of the data suggest a density-dependent parasitoid–host relationship.