Aims: To evaluate the fungitoxic activity of the essential oil of Agastache rugosa alone and to determine its combination effect with ketoconazole against Blastoschizomyces capitatus. Methods and Results: The antifungal activities of the essential oil of A. rugosa and its main constituent estragole were investigated using the broth microdilution, disk diffusion methods and checkerboard microtitre assay. Both estragole and the essential oil exhibited strong activities against the tested fungi and showed synergism with ketoconazole against B. capitatus. Conclusions: Both estragole and the essential oil of A. rugosa have significant growth-inhibiting activities against B. capitatus showing strong synergistic effect with ketoconazole. Significance and Impact of the Study: The essential oil of A. rugosa, combined with ketoconazole, may be particularly useful against B. capitatus, a rare pathogenic fungus documented to cause severe and fatal mycoses in immunocompromised patients.