A 2-yr field study was performed to determine injury and yield response of soybean to long-term O3 exposure and soil moisture deficit. Two levels of soil moisture (well-watered (WW) and water-stressed (WS)) were obtained by differential irrigating during periods of low precipitation. Open-top field chambers were used to expose plants to different levels of O3 for 109 days. The 1983 season was drier and hotter than the 1984 season. In 1983, plants in the (WS) plots were under moderate to severe moisture stress and yielded approximately half as much as those in the (WW) plots. In 1984, plants in the WS plots were under moderate moisture stress during flowering and pod-fill stages and they yielded 20% less than those in the WW plots. In 1983, there was a significant relationship between O3 concentration and yield in the WW plots, but not in the WS plots. In 1984, similar linear responses to O3 occurred at both levels of soil moisture. Compared to the control, predicted yield loss for WW plots at ambient O3 levels (2-yr 7 h per day mean of 0.054 microliters/liter) was 12% in 1983 and 14% in 1984. The predicted loss value for the WS plants in 1984 was 12%.