Bird cherry-oat aphid (BCOA), Rhopalosiphum padi (L.), has been reported to cause yield loss in small grains both through its role as an efficient vector of the PAV strain of Barley yellow dwarf virus (BYDV) and by actual feeding damage to winter and spring small grains by aviruliferous BCOAs. Barley accessions have been reported to have BCOA resistance based on the antibiotic effect of seedlings on the aphids. Whether this antibiosis translates to resistance in terms of grain yield has yet to be shown. Screening for BCOA resistance at the seedling stage has been difficult due to lack of visual symptoms on seedlings infested with BCOA using traditional greenhouse screening methods. In an attempt to develop a seedling screening technique for BCOA we tested many variables including flat type, soil type, infestation date, infestation rate, and screening conditions, (temperature and day length). Traits measured included, leaf number, tiller number, root mass, crown root number, leaf length and width, plant height, fresh weight, dry weight and internode length. In 2005, 78 barleys, reported to be antibiotic to BCOA, were screened with aviruliferous BCOA using traditional seedling screening methods under high temperature and long days and compared to non-infested controls. Seedlings were rated visually on a scale of 1 to 7 (1= resistant and 7= dead). Potential resistant and susceptible checks were identified. In the summer of 2006, a replicated (2X) screening of 5 seedlings each of 960 accessions in the Barley Core Collection was conducted using this technique. Susceptible checks died while resistant checks survived. A total of 1,189 seedlings from 284 accessions survived screening and were transplanted to pots in the greenhouse. Plant height, percent fertile tillers, grain yield and yield components were measured on each plant to verify the rating scale and identify resistance. Results will be discussed.