The present study aims at clarifying the impact of oxidative stress on type B trichothecene production. The responses to hydrogen peroxide (H(2)O(2)) of an array of Fusarium graminearum and Fusarium culmorum strains were compared, both species carrying either the chemotype deoxynivalenol (DON) or nivalenol (NIV). In both cases, levels of in vitro toxin production are greatly influenced by the oxidative parameters of the medium. A 0.5 mM H(2)O(2) stress induces a two- to 50-fold enhancement of DON and acetyldeoxynivalenol production, whereas the same treatment results in a 2.4- to sevenfold decrease in NIV and fusarenone X accumulation. Different effects of oxidative stress on toxin production are the result of a variation in Fusarium's antioxidant defence responses according to the chemotype of the isolate. Compared with DON strains, NIV isolates have a higher H(2)O(2)-destroying capacity, which partially results from a significant enhancement of catalase activity induced by peroxide stress. A 0.5 mM H(2)O(2) treatment leads to a 1.3- to 1.7-fold increase in the catalase activity of NIV isolates. Our data, which show the higher adaptation to oxidative stress developed by NIV isolates, are consistent with the higher virulence of these Fusarium strains on maize compared with DON isolates.