Introduction. The fruit of `Encore' mandarin (Citrus deliciosa × Citrus nobilis) usually shows chlorotic spots in the epicarp from the first stages of development. Thispeel disorder triggers off, thereafter, the occurrence of a large quantity of dark stains (pre-harvest`peel pitting'). Materials and methods. A biochemical and ultra-structural study of the flavedo,using epicarp with and without stains, was carried out. Quality assays were assessed on the edible portionof the fruit or on juice. Results. The cell structure of pitted and unpitted epicarp analysed byelectron microscopy revealed that dark spots were associated with the degradation of cellular membranes.The peroxidase system that shields the epicarp against photooxidative stress showed an inhibition ofcatalase activity and an increase of superoxide dismutase functioning. Moreover, the activities ofascorbate peroxidase and glutathione reductase remained unaffected in stained tissues. In sunshade trees,it was also found that the number of dark stains per fruit decreased but the internal quality of theendocarp was not significantly affected. The weight and the caliber of the fruit also slightly decreased.Discussion. High light intensities induce an oxidative stress in the epicarp, which is characterized by anincreased peroxidation and degradation of biological membranes. On the other hand, protection againstsolar radiation alleviates the stress, but, although not affecting the internal quality of the fruit,slightly decreases the fruit weight and size.