The effect of propionic acid, its sodium salt or a commercial formulation of propionates (0.03, 0.05 and 0.07%), on growth and fumonisin B1 production by Fusarium verticillioides and F. proliferatum isolated was evaluated on irradiated maize at different water activities (aw, 0.93, 0.95, 0.98) and temperatures (15, 25 degrees C). The four isolates grew at all aw x temperature treatments in the absence of propionates. At the highest propionate concentration tested (0.07%), however, growth was restricted to 0.98 aw, for F. proliferatum isolates but not for those of F. verticillioides. Inhibition of growth was maximum when propionates were added in the acid form. In the presence of low propionate concentrations (0.03%), growth was sometimes enhanced probably due to assimilation of these compounds by the fungus. Water activity, temperature, concentration and source of propionate, as well as most two-, three-, four-, and five-way interactions had a significant influence on growth of Fusarium isolates. None of the assayed treatments had any effect on fumonisin B1 production by F. verticillioides isolates. For F. proliferatum, higher fumonisin B1 production occurred in the absence of propionates, and in general concentration decreased with increasing doses of preservatives. Single factors (aw, propionate concentrations and temperature) and temperature x aw and propionate concentration x temperature interactions had a significant effect on fumonisin production (p < 0.01). Moreover, propionate concentration was the single most important factor, besides temperature, which affected fumonisin B1 production.