Seven plant species were exposed in open-top chambers to four levels of ozone (O₃) during two growing seasons and screened for treatment effects on the fast chlorophyll a(Chl) fluorescence transient kinetics of dark-adapted leaves, and on the fluorescence signals obtained from the same leaves in illuminated steady-state. The aim was to identify the nature of O₃ effects on PSII, and to determine inter-specific differences. In dark-adapted leaves, O₃ caused a reduction in variable fluorescence (F_V : F₀), indicating an overall reduction in the efficiency of primary photochemistry. A large increase in excitation energy dissipation per active reaction centre (DI₀/RC) and a smaller increase in the trapping rate of excitons (TR₀/RC), showed that a fraction of the reaction centres was inactivated while the rest sustained full functionality. The magnitude of the effect increased in the order of Bromus erectus Centaurea jacea Trisetum flavescens Rumex obtusifolius Plantago lanceolata Trifolium pratense Knautia arvensis. The inter-specific variability in PSII responses could not be explained solely by specific differences in modelled O₃ uptake by the leaves. Visible leaf injury was not related to changes in fluorescence emission. In illuminated steady-state, O₃ sensitivity was most expressed in the change in quantum yield of photosynthetic electron transport (Φ_PSII). The ranking of species differed from the ranking obtained in dark-adapted leaves. These results suggest that the mechanistic basis for O₃ effects on PSII is similar in all species, but that inter-specific differences exist in the magnitude of change which cannot be explained solely by different O₃ uptake rates. The observed changes in fluorescence signals are not O₃-specific.