6-Arylamino-phthalazine-5, 8-diones and 6, 7-bis (arylthio)-phthalazine-5, 8-diones were synthesized and tested for in vitro antifungal activity against two pathogenic strains of fungi. Among those tested, many compounds showed good antifungal activity. The results suggest that phthalazine-5, 8-diones would be potent antifungal agents.