Isoalantolactone,a major constituent of Inula racemosa (Compositae) was tested for its antimicrobial action against five bacteria,six human and six plant pathogenic fungi. The lactone showed absolute toxicities at 500 mg/mL against 3 soil borne phytopathogenic fungi (Gaeumannomyces graminis var. tritici,Rhizoctonia cerealis and Phytophthora capsici) with the MICs determined to be 100,100 and 300 mg/mL,respectively. At the MICs,isoalantolactone exhibited its fungistatic nature of toxicity. The lowest fungicidal concentrations of the lactone to G. graminis var. tritici,R. cerealis and P. capsici were shown to be 150,150 and 350 mg/mL,respectively. Moreover,isoalantolactone displayed weaker antibacterial activities against Bacillus subtilis,Escherichia coli,Pseudomonas fluorecense,Sarcina lentus and Staphylococcus aureus with MICs of 125,425,150,150 and 100 mg/mL,respectively. The investigation disclosed the strong inhibition of isoalantolactone to the phytopathogenic fungi,raising a possibility that the lactone could be considered as a starting point for the project aiming at the development of new fungicide(s).