Citation: Proctor, R., Mccormick, S.P., Alexander, N.J. 2009. Variation in the Trichothecene Mycotoxin Biosynthetic Gene Cluster in Fusarium [abstract]. American Chemical Society. Technical Abstract: Trichothecene mycotoxins are produced by some plant pathogenic species of the fungus Fusarium and can contribute to its virulence on some plants. In Fusarium graminearum and F. sporotrichioides trichothecene biosynthetic enzymes are encoded at three loci: the single-gene TRI101 locus; the two-gene TRI1/TRI16 locus; and the 12-gene, core TRI cluster. Here, DNA sequence analysis revealed that TRI1 and TRI101, but not TRI16, are located in the core cluster in a third species, F. equiseti. By examining variation in TRI1, TRI16, TRI101, core TRI cluster genes, and primary metabolic genes, we obtained evidence that TRI1 and TRI101 originated outside the core cluster and moved into it during the evolution of F. equiseti and closely related species. We also identified four types of TRI1 orthologues and obtained evidence that three of the orthologue types catalyze trichothecene C7 and C8 hydroxylation and that the fourth catalyzes C8 hydroxylation only.