Rhopalosiphum padi L., or the bird cherry-oat aphid, causes significant damage to winter wheat (Triticum aestivum L.) in the Great Plains. Our objective was to develop a juvenile-plant bioassay for BCO tolerance that allows rapid phenotypic characterization of tolerance in a growth chamber study using root and shoot weight measurements of 3-wk old seedlings produced in seed germination pouches. Based on preliminary results, bioassay methods were used in verification experiments conducted on one hundred and forty-nine Oklahoma, Colorado, SRPN, and CIMMYT lines indicated levels of responses to feeding (roots being greater than shoots), but both averaging around a 30 % reduction. Findings of indigenous genetic variation in six of the eight control treatments (P>0.05) further exonerated the need for control plants to curve variation by a ratio of infested-to-control. Correlation relationships between three of the four sources showed that both roots and shoots are key to finding tolerance.