Flue-cured tobacco (Nicotiana tabacum McNair 944) was exposed to chronic doses of ozone (O3) in open-top field chambers to determine the influence of frequency and magnitude of peak O3 concentrations and daily exposure duration on tobacco yield response. The treatments were established by adding O3 in amounts that were proportional to ambient O3 concentrations or in constant amounts. The frequency of occurrence and level of peak O3 concentrations were greater for each proportional-addition treatment than for the corresponding constant-addition treatments. However, the seasonal mean O3 concentrations were nearly identical, and the yield response to O3 was similar for both types of O3 for 12 hr/day (1000 to 2200 hours EDT) was 10% less (three treatment levels combined) than in those receiving proportional addition for 7 hr/day (1000 to 1700 hours EDT). If other important crop species have a similar response to O3 late in the afternoon, previous national crop loss estimates based on seasonal 7 hr/day O3 exposures may be low.