Extracts of Chaetomium globosum EF18, isolated as endophytic fungus from Withania somnifera, were found effective against Sclerotinia sclerotiorum. Ethyl acetate and methanol extracts were more effective than hexane extract showing >80% growth inhibition. Bioactive compound (antibiotic Sch 210971, m/z 445 and λ max 290) having antifungal activity against S. sclerotiorum has been isolated in pure form from the ethyl acetate extract following bioassay guided fractionation. Apart from this compound other fractions of polar to medium polarity were also found effective. Fraction no. VIII from VLC (Vacuum liquid chromatography) column of ethyl acetate extract was most active having IC 50 value 35.4 μg/ml.