To study the effects of a low concentration of ozone on growth and gas exchange in Eucalyptus globulus Labill. seedlings, ozone was applied for 37 days at a concentration of 50 ppb for 7 h daily under conditions of low light (250 µmol m–2 s–1 (PAR)) and controlled temperature (20 °C). The seedlings exhibited extreme sensitivity to ozone. The ozone treatment reduced total plant biomass but had no effect on the partitioning of assimilate. Photosynthesis, stomatal conductance and internal CO2 concentration were all reduced by ozone. The decline in photosynthesis was partly the result of direct effects of ozone on the stomata.