Ozone damage to plants has been attributed to the action of oxygen free-radicals and other ozone degradation products against which cellular antioxidant systems have been considered to be a front-line defense. The activity of superoxide dismutase (SOD), one such antioxidant, has been shown to increase in ozonated plants. Past work with pea (Pisum sativum L.) in our laboratory showed that the cytosolic Cu/Zn-SOD isoform and its transcript were most responsive to ozone, compared to chloroplastic Cu/Zn-SOD. In the current work we tested the hypothesis that plants that constitutively overexpress cytosolic SOD are more tolerant of ozone. Pea cytosolic Cu/Zn-SOD was overproduced in the cytosol of two cultivars of transformed tobacco (Nicotiana tabacum), Bel W3 and Wisconsin 38. Young and recently expanded leaves of transgenic plants of both cultivars showed less foliar necrosis than nontransformed controls when exposed to acute doses of ozone. We suggest that this may demonstrate the importance of Cu/Zn-SOD in the cytosol as a protector of the integrity of the plasma membrane and possibly other cellular constituents.