Assessments of ozone sensitivity in tree species have been based primarily on studies of Seedlings without any knowledge of the similarities between seedling and mature tree responses. To determine if seedlings and trees responded similarly to ozone, 2-yr-old seedlings and 30-yr-old trees of Quercus rubra L. were fumigated with subambient, ambient and twice-ambient ozone for one growing season (cumulative doses of 18, 45 and 87 ppm h−1, respectively). Carboxylation efficiency, apparent quantum yield and light-saturated net photosynthesis of leaves from the lower canopy of the 30-yr-old trees were reduced by ambient and twice-ambient ozone treatment. However, physiological characteristics of leaves from the upper canopy of trees showed no signs of ozone injury. Canopy leaf dry weight of trees was reduced by the twice-ambient ozone treatment through reductions in leaf area rather than specific leaf weight, but bole diameter and branch growth of trees were unaffected by ozone treatment. In contrast, ozone did not influence leaf physiology of second flush leaves, stem diameter, and height and biomass growth of 2-yr-old seedlings. These results indicate that for deciduous hardwood species, studies of seedlings may underestimate the ozone sensitivity of larger and more physiologically mature trees. For large trees with developed canopies, leaf position within the canopy may influence the sensitivity of physiological processes to ozone.