Grain sorghum(sorghum vulgare Pers.) is an important animal feed crop, and it is sometimes planted as a substitute for field corn. Although sorghum is grown in areas of the central and southern U.S. where potentially damaging concentrations of 03 exist, no data are available regarding the sensitivity of grain sorghum to 03. Plants of grain sorghum (DeKalb A28+) were field-grown in open-top chambers and exposed to 03(7-hr/day seasonal mean concentrations of 0.016, 0.040, 0.059, 0.078, 0.102, and 0.129 ppm) for 85 days to determine the impact of 03 on grain yield. A randomized complete block design incorporated three replicates of all treatments. Foliar injury was noted at the two highest 03 treatments. Analysis of variance of the data indicated highly significant 03 effects on overall grain yield.