Technical Abstract: -Mean surface ozone concentration is predicted to increase 25% by 2050. Previous chamber studies of crops report large yield losses due to elevation of tropospheric ozone, and have been the basis for projecting economic loss. -This is the first study with a food crop (soybean) using Free-Air gas Concentration Enrichment (FACE) technology for ozone fumigation (www.soyface.uiuc.edu). A 23% increase in ozone concentration from an average daytime ambient 56 p.p.b. over two growing seasons decreased seed yield by 20%. -Total aboveground net primary production decreased by 17% without altering dry mass allocation among shoot organs, except seed. Fewer live leaves and decreased photosynthesis in late grain filling appear to drive the ozone-induced losses in production and yield. -These results validate previous chamber studies suggesting that soybean yields will decrease under increasing ozone exposure. In fact, these results suggest that when treated under open-air conditions yield losses may be even greater than the large losses already reported in earlier chamber studies. Yield losses with elevated ozone were greater in the second year following a severe hailstorm, suggesting that losses may due to ozone may be exacerbated by extreme climatic events.