Previous work indicated that cinnamon was inhibitory to the growth of aflatoxin-producing molds. The objective of this study was to determine the specific components of cinnamon and cloves that may be effective against mold growth and toxin production. The effects of cinnamon oil, clove oil, cinnamic aldehyde and eugenol on growth and aflatoxin production by Aspergillus parasiticus were studied using yeast-extract sucrose broth as the substrate. All four substances inhibited mold growth and subsequent toxin production. Cinnamon and clove oils were inhibitory at 200–250 ppm, cinnamic aldehyde at 150 ppm and eugenol at 125 ppm. Since cinnamic aldehyde and eugenol are the respective major components of cinnamon and clove oils, it was concluded that these are the major active antifungal ingredients of these two essential oils. The inhibitory effect of these substances was judged to be inhibition of growth rather than of toxin production. When growth occurred after a delay, aflatoxin production occurred when the cultures reached secondary metabolism. Given sufficient time, cultures which were inhibited initially, but which subsequently grew, produced toxin levels equivalent to control cultures. Levels of the oils above 250 ppm and of cinnamic aldehyde and eugenol above 200 ppm completely inhibited mold growth, or permitted only a small amount of growth that never reached secondary metabolism and never produced aflatoxins during the time of this study.