Because the current critical level of ozone (O₃) for forest trees is based only on one species, the responses of five deciduous tree species were differentiated in a climate chamber experiment. The number of symptomatic leaves per tree was significantly increased, and stomatal conductance was decreased under 50% ambient+30 nl l^-1 O₃ as compared to 'normal' senescence at 50% ambient [O₃]. Species with a high stomatal conductance did not show earlier or more leaf injury symptoms. The additional 30 nl l^-1 O₃ induced specific pectinaceous cell wall protrusions, phenolic cell wall incrustations, tonoplast vesicles, and inhomogeneous, condensed/precipitated phenolic material in the vacuoles. Due to added O₃, cell senescence was accelerated with increased electron-density of the cytoplasm, and initial chloroplast degeneration. The slow degeneration process started in mesophyll cells, and expanded into epidermal and finally guard cells. Because of the large variance in biomass between individuals and species, the current critical level is supported by the assessment of visible leaf symptoms rather than growth reduction.