Sclerotinia minor accumulates β-carotene at levels dependent upon oxidative growth conditions and differentiation. β-carotene accumulation is 2.5-fold higher in differentiated mycelia at high than at low oxidative stress, and approx. 3-fold higher in differentiated than in undifferentiated mycelia. It is proposed that β-carotene may be produced by the fungus to counteract oxidative stress that develops during growth. This is shown by the finding that exogenous β-carotene at growth non-inhibiting concentrations causes a concentration-dependent reduction of oxidative stress (lipid and protein peroxidation) and sclerotial differentiation in this fungus. The data of this study support our hypothesis that sclerotial differentiation in phytopathogenic fungi may be induced by oxidative stress.