The effect of natural phenolic acids from wheat bran on type B trichothecene biosynthesis by Fusarium culmorum was investigated in vitro. Durum wheat bran contained various monomeric forms of phenolic acids, with ferulic acid being the most abundant. In addition, various oligomeric forms of ferulic acid and mainly dimeric forms were also detected. When liquid cultures of F. culmorum were supplemented with a natural wheat bran extract, trichothecene production was fully inhibited. The exact mechanism by which toxin synthesis is repressed remains to be clarified but we showed that the phenolic acid treatment resulted in a drastic reduction in the expression level of structural trichothecene biosynthetic genes. The inhibitory efficiency of the natural phenolic acid extract was significantly higher than that of a reconstituted mixture containing similar concentrations of monomeric forms. Thus, to elucidate the full repression of type B trichothecene production induced by the natural phenolic acid extract from wheat bran, two hypotheses can be raised: (i) a synergistic impact of monomeric and dimeric forms of phenolic acids, (ii) the occurrence of an unidentified oligomeric form able to efficiently repress toxin yield. As a first attempt to investigate the effect of oligomeric forms, one of the most abundant dimer of ferulic acid, the 8-5′-benzofuran dimer, has been synthesized in vitro and was shown to inhibit trichothecene biosynthesis to the same extent than the monomer of ferulic acid.