Spring wheat, Triticum aestivum L., cv. Drabant, was exposed to different concentrations of ozone in open-top chambers for one growing season, at a site located in south-west Sweden. The chambers were placed in a field of commercially grown spring wheat. The treatments were charcoal-filtered air (CF), non-filtered air (NF), non-filtered air + 25 nl 1−1ozone (NF +) and non-filtered air + 35 nl 1−1ozone (NF++). Ozone was added daily between 11.00 and 18.00 h, starting at anthesis. Fumigation with elevated concentrations of ozone caused chlorosis of the flag leaves. In the NF++ and NF+ treatments chlorosis appeared after 7 and 20 days of fumigation, respectively, and in the CF treatment after about 34 days, A progressive series of changes occurred in the mesophyll cells of the flag leaves after ozone fumigation. Starting at anthesis, the first changes occurred in the cytoplasm and involved a decrease in the amount of cytoplasm and an increase in the vacuolization of the cells. Subsequently the chloroplasts became affected decreasing in area and containing more plasto-globuli. Finally vesicles were formed between the grana thylakoids and at the same time the plasma membrane came loose from the cell wall and convoluted. The mitochondria remained unaffected until only lipid droplets, oleosomes, were left in the cells. These changes were not specific for ozone as they occurred also in the CF treatment, although later in time. This indicates that ozone causes premature senescence in wheat flag leaves.