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St;MMARYClonal birch (Betula peiidula Roth.) seedlings at three different de\ elopmental stages were fumigated for 12 h daily with 50, 90 and 1 30 ppb ozone for 25 d. When transferred to fumigation chambers, the leaves of plant group 1 were almost expanded, in group 2 they were fast enlarging, and in group 3 all the leaves emerged under the exposure. The plants in which the lea\es emerged under the ozone stress were the most resistant to ozone. In groups 1 and 2 ozone treatments reduced the growth of leaf biomass. and induced more visible injuries on leaves and ultrastructural symptoms in chloroplasts than they did in group 3. The stomatai densjt> of leaves increased in response to ozone in all the groups, Tbe effects of ozone on stomatal conductance were complex. In groups 1 and 2 the 50 ppb ozone exposure increased stomatal conductance whereas lowered conductances were observed in group 3 after fumigation with 90 and 130 ppb ozone. The results suggest that the leaves developing under ozone fumigation were better able to tolerate tbe ozone stress, as indicated by unaffected or stimulated growtb and small amounts of \-isible and ultrastructural injuries and stomatal cbanges. It is possible to regard the changes as acclimation reactions.