A naturally occurring phenylpropanoid, trans-anethole (anethole), was assayed for its fungicidal activity in Saccharomyces cerevisiae grown under hypoxic and aerobic conditions. Anethole killed the growing cells in malt extract broth only under hypoxic conditions. Anethole did not exhibit fungicidal effects against non-growing cells. The cells of a mitochondrial-DNA-lacking rho0 mutant, which grew fermentatively even in the presence of O-2, were killed by anethole regardless of aeration. The fungicidal potency of anethole against a strain capable of respiration under aerobic conditions was enhanced by the addition of a respiratory inhibitor antimycin A(3). Therefore, anethole possibly expresses fungicidal activities only against fermentatively growing cells of S. cerevisiae.