Plant response (e.g., chlorosis and leaf rolling), density of Russian wheat aphid, Diuraphis noxia (Mordvilko), and relative abundance of parasitoids (Hymenoptera: Braconidae and Aphelinidae) were measured on 20 lines of field-grown barley, Hordeum vulgare L., varying from highly resistant to highly susceptible to Russian wheat aphid. Barley lines were challenged with Russian wheat aphid throughout plant development. Plant and insect responses were compared with results from greenhouse tests which assessed seedling resistance. Ratings of resistance based on chlorosis from previous greenhouse seedling tests predicted plant susceptibility to Russian wheat aphid in later plant growth stages as measured by chlorosis and leaf curling, 67 d after planting. Russian wheat aphid were found on all barley lines, and densities increased as seedling plant susceptibility increased, resulting in densities as high as 70 aphids per tiller. Parasitoids detected were Diaeretiella rapae McIntosh (Hymenoptera: Braconidae); Aphelinus albipodus Hayat & Fatima, A. varipes (Foerster), and A. asychis (Walker) (Hymenoptera: Aphelinidae). Weighted parasitoid abundance (count of aphid mummies per tiller ÷ count of live Russian wheat aphids per tiller) for D. rapae averaged up to 0.75:1 among the barley lines. For the aphelinids, weighted parasitoid abundance was typically lower, but in 1 late-season occurrence, a ratio of 6:1 was detected (in this instance parasitoids were released around the perimeter of the experiment). In general, 5 times as many A. albipodus were detected as A. asychis, even though they were released in equal numbers. Weighted parasitoid abundance varied less among the barley lines than plant damage and aphid densities.