The effect of modified atmospheres on the growth and fumonisin B 1 production of Fusarium verticillioides and Fusarium proliferatum on corn is presented in a series of two papers. In this, the first part, the effect of initial headspace (IH) carbon dioxide concentration and its interaction with water activity (a w ) on growth and fumonisin B 1 production was evaluated. It was observed that at all a w values studied, increase in the IH CO 2 concentration generally resulted in a decrease in the colony growth rate (g, mm day −1 ) and maximum colony diameter (D max , mm) and an increase in the lag phase duration (λ, day). Although both a w and IH CO 2 concentration had significant and synergistic effects on g, a w had the largest effect. As little as 10% IH CO 2 completely inhibited the production of fumonisin B 1 by F. verticillioides. F. proliferatum was more resistant and required 40, 30 and 10% IH CO 2 at a w 0.984, 0.951 and 0.930, respectively, to completely inhibit fumonisin B 1 production. These results demonstrate that modified atmospheres containing high CO 2 levels could potentially be employed for the protection of corn from fungal spoilage and mycotoxin contamination during the post-harvest period.