Hybrid poplar (Populus trichocarpa $ imes$ P. deltoides) clones were fumigated with ozone. Fumigation was applied for 6 to 8 hours each day for approximately 3 months at ozone concentrations of 0.090 to 0.115 ppm using by open-top chambers. Growth and biomass of hybrid poplar seedlings were reduced by ozone exposure. Mean percentage of falling leaves in ozone-treated plant was 6 times higher than that of charcoal-filtered plant. Among physiological responses, rate of photosynthesis, stomatal conductance, transpiration and initial Rubisco activity were significantly lower in seedlings grown in ozone environment compared seedlings grown in charcoal-filtered air. All these physiological results supported that biochemical process to be a key feature to understand reduction in photosynthesis.