The essential oils of garlic and onion and their constituent sulfides with three or more sulfur atoms were potent inhibitors of yeast growth. The minimum inhibitory concentrations of garlic oil, onion oil, diallyl trisulfide, diallyl tetrasulfide, and dimethyl trisulfide for all the yeasts tested ranged between 2 and 45 ppm. The oils and their constituent sulfides, however, were only very weakly antibacterial, showing minimum inhibitory concentrations of greater than 300 ppm for most of the bacteria tested. The antiyeast activity of garlic oil and onion oil was storage stable and was not influenced by pH. Film formation on soy sauce by Zygosaccharomyces rouxii SS1 was completely prevented for 30 days by the addition of 30 and 40 ppm of garlic oil and onion oil, respectively.