The impact of ozone fumigation on chlorophyll a fluorescence parameters and chlorophyll content of birch trees grown at high and low fertilization were studied for 6-, 8-, and 12-week old leaves. Fluorescence parameters were measured with a portable fluorometer with its fibre optics tightly inserted in a gas exchange cuvette at light intensities from 0 to 220 μmol photons m^-2 s^-1. Ozone caused significant changes of primary photosynthetic reactions: a decrease of the quantum yield of photosystem II and an increase of non-photochemical quenching. In all leaves a biphasic light response of nonphotochemical quenching was observed. Ozone fumigation shifted the onset of the second phase from a PFD of about 60 μmol m^-2 s^-1 to about 30 μmol m^-2 s^-1. While the fertilizer concentration had no influence on this character, high fertilization supply of plants partially reduced O-3-induced damage. The light responses of Ft, Fm' and NPQ observed in birch leaves grown in O-3-free air indicate the existence of at least two different processes governing energy conversion of the photosynthetic apparatus at PS Il in the range of PFD 0-200 μmol photons m^-2 s^-1 The first phase was attributed to a rather slowly relaxing type of non-photochemical quenching, which, at least at low PFD, is thought to be related to a state 1-2 transition. The further changes of the fluorescence parameters studied at higher PFD might be explained by an increase of energy-dependent quenching, connected with the energization of the thylakoid membrane and zeaxanthin synthesis. A major effect of ozone treatment was a lowering of PS II quantum yield. This reflects a reduction of PS II electron transport and corresponds to the reduction of CO₂-fixation observed in ozonated leaves.