The study reported was conducted to establish the impacts of photochemical oxidants (primarily ambient ozone, O3) on the yield of watermelon (Citrullus lanatus) at a site on the east coast of Spain. Fruit yield and quality were monitored in plots established in a commercial watermelon field exposed, in open-top chambers (OTCs), to non-filtered air (NFA; near-ambient levels of ozone) or charcoal-filtered air (decreased levels of photochemical oxidants including O3; CFA), or to ambient air (AA), during the 1988 and 1989 growing seasons. Ambient levels of O3 were found to exceed present UN-ECE (United Nations Economic Commission for Europe; Convention on Long-Range Transboundary Air Pollution) critical level guidelines for the protection of crop yield by approx. twofold in 1988 and by approx. fivefold in 1989. Plants exposed to NFA and AA developed visible O3 injury on the upper surface of sun-exposed older leaves, and fruit yield (annual marketable fruit weight and number) was found to be depressed in OTCs ventilated with NFA in comparison with those receiving CFA. Consistent with inter-annual variations in O3 exposure, greater yield losses were experienced in 1989 (39%) than in 1988 (19%), an effect mediated predominantly by a decline in fruit number rather than average fruit weight. Exposure to ambient levels of ozone also slightly decreased fruit quality (4–8% decline in soluble solids content). Leaf gas exchange measurements made in the field in 1988 revealed effects of O3 on fruit yield and quality to be associated with a decline in the net CO2 assimilation rate per unit leaf area under light saturation (Asat) and stomatal conductance to water vapour (gs), and enhanced rates of dark respiration. A/ci curves (where A is the net CO2 assimilation rate per unit leaf area and ci is the mole fraction of CO2 in the leaf intercellular air space) constructed for plants grown in laboratory-based closed chambers, and exposed to an accumulated O3 exposure similar to that experienced by plants in the field, suggested that the likely cause of the decline in photosynthetic capacity was (1) a decrease in the amount and/or activity of Rubisco and (2) an impaired capacity for regeneration of ribulose 1,5-bisphosphate, which was not mediated through changes in the photochemical efficiency of photosystem II (Fv/Fm, where Fv is variable chlorophyll a fluorescence and Fm is maximum chlorophyll a fluorescence). No shift in the relative stomatal limitation to photosynthesis was observed under the influence of O3, suggesting that the decline in gs induced by the pollutant in both field and laboratory was the result, and not the cause, of the decrease in Asat. Ozone exposure also caused a decrease in C isotope discrimination (approx. 0.5‰), a shift that revealed a departure from predicted theory based on supporting leaf gas exchange measurements. The study demonstrates that ambient levels of photochemical oxidants on the Spanish Mediterranean coast are high enough to adversely influence the yield and physiology of an economically important crop grown in the region, and the magnitude of the effects was greater than would be predicted from exposure–yield-loss relationships for grain and fodder crops in central and northern Europe. Implications for the derivation of UN-ECE ozone standards are discussed, along with the mechanistic basis of the observed yield decreases.