‘Tiny Tim’ tomato plants were exposed to five CO2 treatments (375 (ambient), 450, 525, 600 or 675 μmol mol−1) in combination with O3 (0 or 80 nmol mol−1). Biomass was evaluated following 3, 5, 7 and 13 wk exposure. Biomass following 13 wk exposure also included weekly harvests of mature tomato fruit beginning week 8. Carbon dioxide enrichment significantly enhanced total vegetative plant d. wt at each harvest, as well as cumulative yield of mature fruit, whereas O3 significantly suppressed total vegetative plant d. wt at each harvest and reduced total cumulative fruit yield. The magnitude of these changes varied with the development of tomato from early growth to mature fruit yield. Carbon dioxide enrichment reduced the detrimental effects of O3 on total vegetative plant d. wt of tomato following 3, 5, 7 and 13 wk exposure. Final mature fruit yield was 24% higher under enriched CO2 treatments than in ambient CO2. Ozone suppressed final yield by 31% following exposure to 80 nmol mol−1 O3 when compared with exposure to charcoal-filtered (CF) air. The impacts of both CO2 and O3 on yield were, however, dependent upon the presence or absence of the other gas. In the absence of O3, yields were very similar for the ambient and elevated CO2 treatments, but in the presence of O3, yields under ambient CO2 were greatly suppressed whereas yields under elevated CO2 were similar to those in the absence of O3. Thus, enriched CO2 ameliorated most of the suppressive effect of O3 on yield of mature fruit.