Field corn(Zea mays L.) is the most important agricultural crop in the U.S. and the major production areas are subjected to potentially damaging concentrations of ozone (O3). Since no information was available regarding the sensitivity of field-corn hybrids grown in the Midwest, plants of two hybrids were field grown in open-top chambers and exposed to O3 for 83 days to determine its impact on grain yield. The air chamber was either charcoal filtered or non-filtered with various constant amounts of O3 added to the ambient air from 0900 to 1600 CST. Also included was a nonchambered plot. A randomized complete block design incorporated 3 replicates of 7 treatments. Foliar injury was noted on some plants at the elevated O3 concentrations but was never more than 5 percent on an individual leaf. Analysis of variance of the data indicated significant ozone effects on overall grain yield, 100-seed weight, seeds per cob, cob weight, and cob length, for both hybrids. (Copyright (c) Canadian Journal of Botany, December 1985.)