The performance of four isogenic lines of ‘Atlas’ barley (Hordeum vulgare L.) ‐ ‘Full‐,’ ‘Half‐,’ ‘Quarterawned,’ and ‘Awnless’ ‐ was measured at 22 locations in western U.S. and Canada, and in a date‐of‐planting experiment at Davis, Calif. The relative mean yields over all environments were 110.1, 112.6, 105.1, and 100.0 for Full‐, Half‐, Quarter‐awned and Awnless, respectively. Although Half‐awned had the highest mean yield over all locations, a differential trend in genotype performance at high and low yield levels was evident. Half‐awned was most productive at low yield levels and was superior to Awnless and Quarter‐awned throughout the range of productivity. Full‐awned was the highest‐yielding in good environments, but lower than Half‐awned and comparable with Quarter‐awned in low‐yielding environments. Isogenic line‐environment interactions were not strongly associated with years or locations, but were generated by conditions specific to each test site and were related, in part, to the productivity level of the site.The differential performance of Full‐awned in the low and high‐yielding environments was independent of kernel weight. Competition for substrate at the time of awn development and initiation and development of additional florets and tillers was suggested as a possible cause for the failure of Full‐awned to attain maximum yield expression in low‐yielding environments.The linear regression coefficient of kernel weight on awn length, b = 0.69 mg kernel−lcm−l, obtained in the date of planting study, agreed very closely to the value (0.64) reported previously. The contribution of the awns to grain yield was estimated as 35.6 kg ha−1cm,−l. In addition to the effect on kernel weight, the awns provided considerable protection to the developing kernels from infection by foliar pathogens.