Essential oils from oregano and thyme were applied for 24 h as fumigants against the mycelia and spores of Aspergillus flavus , Aspergillus niger and Aspergillus ochraceus , as well as against natural microflora of wheat grains. The minimal inhibitory concentration (MIC) of oregano oil needed to inhibit the mycelial growth of the fungi was 2.0 μl/L, while spores were eradicated following exposure to 2.0 to 2.5 μl/L. The thyme essential oil was less efficient in controlling mycelia and growth was observed even following exposure to 4.0 μl/L. However, the thyme essential oil was fungitoxic to spores (MIC = 3.0 μl/L). In another set of trials the efficacy of the oils and two of their constituents (carvacrol and thymol) in controlling natural microflora of surface-sterilized wheat grain was studied. Of the four materials investigated, only oregano essential oil exhibited fungicidal activity and, following 24 h exposure to 20 μl/L, a significant reduction in the percent of infested grain was observed even after 5 days of incubation on potato dextrose agar. A reduction in the germinability of the grains was evident following exposure to the materials tested. When the fungicidal activity of oregano essential oil was evaluated using grains with different moisture contents (MC), data revealed that the better inhibitory effect was achieved in grain with a high MC. The findings emphasize the toxicity of oregano and thyme essential oils as fumigants against fungi attacking stored grain and strengthen the possibility of using them as an alternative to chemicals for preserving stored grains.