In previous studies with tomato (Lycopersicon esculentum L.) exposed to a low level (3.7 kJ m –2) of UV-C (λ: 254 nm) radiation, which is defined as a beneficial level, we report a delay in fruit ripening by at least 1 week for treated fruit. In the present study, we investigate the changes in the activities of different enzymes involved in defense mechanisms, such as guaiacol peroxidase, ascorbate peroxidase, catalase, superoxide dismutase, ascorbate oxidase, lipoxygenase and phenylalanine ammonia lyase in tomato fruit in response to a beneficial level of UV-C. The irradiation leads to an increase in the guaiacol peroxidase and ascorbate peroxidase activities, whereas catalase activity remains similar to the control. The activities of superoxide dismutase and ascorbate oxidase were significantly reduced after UV-C exposure. In UV-C-treated fruit, an increase of lipoxygenase and phenylalanine ammonia lyase activities occurred within the first 5 d, followed by a second period in which these activities were below those of the control. Our study suggests that the level of UV-C used induced a rapid but moderate accumulation of photooxidation products, to which plants react by stimulating their defence mechanisms against oxidation. This activation may explain the delay observed in ripening and senescence of irradiated tomato fruit.