This fiel experiment was carried out to investigate the physiological changes following water stress in bell pepper plants and their consequences on fruit yield. Bell pepper plants were grown under irrigated and rainfed conditions. Water availability to plants was assessed by measurements of leaf water potentials. Growth was determined by measuring plant (above-ground) and fruit dry-mass accumulation. Solar radiation intercepted by the crop was measured with a linear net radiometer. Leaf protein content and ion accumulation in leaves and fruits were biochemically determined. Leaf photosynthesis and stomatal conductance were measured and mesophyll conductance was estimated in vivo during the crop cycle under field conditions using a portable gas analyser. The quantum yields of the two photosystems were estimated by measurements of chlorophyll ¯uorescence and absorbance (at 830 nm), in rapid succession. Dry-mass accumulation in above-ground plant organs, gas-exchange characteristics, leaf water potentials, and intercepted solar radiation were lower in rainfed plants than in the irrigated control. Leaf ion and protein concentrations were higher in rainfed than in irrigated plants, while ion accumulation in fruits did not change between treatments. The correlation between the quantum yield of PSI and PSII was similar in irrigated and rainfed plants and did not change even under severe water stress. This experiment showed that photosynthetic limitations in plants growing in the ®eld and subjected to moderate stress conditions were mainly due to decreasing stomatal conductance. In rainfed conditions leaf photosynthesis was also impaired by the reduction of mesophyll conductance, as water stress became severe. The photochemical efficiency of bell pepper leaves was however resistant to drought stress. Photosynthetic limitation was sufficient to reduce plant growth, fruit yield and fruit marketability. This indicates that irrigation is necessary to optimize bell pepper production, and that deficit irrigation may postpone the decline in gas exchange and plant growth as soil becomes dry.