The effect of natural phenolic compounds (vanillic and caffeic acid) and water activity (a(w)) on the growth and mycotoxin production on corn by Fusarium and Aspergillus isolates was investigated. Large differences were observed in the response of the Fusarium and Aspergillus isolates to the phenolic compounds, mostly determined by their genus. Generally for both F. verticillioides and F. proliferatum, an increase in concentration of either caffeic or vanillic acid resulted in a decrease in the colony growth rate and increase in the lag phase duration. Growth of the Fusarium isolates was not completely inhibited at the highest a(w) value evaluated of 0.967, with complete inhibition only being observed at combinations of high phenolic acid concentrations (> or =2000 microg g(-1)) and low a(w) values (0.948). Within the experimental limits investigated, growth of the Aspergillus species was not affected by the phenolic compounds. Application of the phenolic compounds significantly reduced fumonisin B(1) and aflatoxin B(1) production. Although the effects noted in artificial media appear to be carried over to corn, rather high concentrations are required to observe similar effects on corn and to completely inhibit growth where possible. This implies the occurrence of interactions of the phenolic compounds with some matrix components which may reduce their overall effectiveness.