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Aims:To determine the effect of oxidative stress and exogenous b-carotene on sclerotial differentiation and carotenoid yield of Penicillium sp. PT95. Methods and Results: In this experiment, high oxidative stress was applied by inclusion of FeCl 3 (10 lmol l )1 ) in the growth medium and by light exposure. Low oxidative stress was applied by omitting iron from the growth medium and by incubation in the dark. Supplementation of exogenous b-carotene (as antioxidant) to the basal medium caused a concentration-dependent delay of sclerotial differentiation (up to 72 h), decrease of sclerotial biomass (up to 43%) and reduction of carotenoid yield (up to 92%). On the contrary, the exogenous b-carotene also caused a concentration-dependent decrease of lipid peroxidation in colonies of this fungus.Conclusions: Under high oxidative stress growth condition, the sclerotial biomass and carotenoid yield of PT95 strain in each plate culture reached 141 mg and 30AE03 lg, which were 1AE53 and 3AE51 times higher respectively, than that at low oxidative stress growth condition. Significance and Impact of the Study: These data prompted us to consider that in order to attain higher sclerotial biomass and pigment yield, the strain PT95 should be grown under high oxidative stress and in the absence of antioxidants.