Effect of exogenous H2O2 and catalase was tested in liquid cultures of the deoxynivalenol and 15-acetyldeoxynivalenol-producing fungus Fusarium graminearum. Accordingly to previous results, H2O2 supplementation of the culture medium leads to increased toxin production. This study indicates that this event seems to be linked to a general up regulation of genes involved in the deoxynivalenol and 15-acetyldeoxynivalenol biosynthesis pathway, commonly named Tri genes. In catalase-treated cultures, toxin accumulation is reduced, and Tri genes expression is significantly down regulated. Furthermore, kinetics of expression of several Tri genes is proposed in relation to toxin accumulation. Biological meanings of these findings are discussed.