Fumonisin mycotoxins are produced by Gibberella fujikuroi (Fusarium moniliforme) mating population A, a major pathogen of maize and sorghum worldwide. Fumonisin biosynthetic genes are being identified by genetic crosses utilizing naturally occurring fumonisin production variants. Meiotic analysis has identified three putative fumonisin biosynthetic loci. Fum1, which can control the ability to produce fumonisins, is being localized by marker-based mapping. Fum2 and fum3, which control hydroxylation of carbon-10 and carbon-5, respectively, appear to be linked. Additional experimental crosses should elucidate the linkage relationships among fum1, fum2 and fum3. When genetic analysis has localized the position of the fumonisin biosynthetic genes to a particular chromosomal region or regions, the genes will be identified by complementation of function via DNA-mediated transformation. Understanding fumonisin biosynthesis and its regulation should facilitate development of measures to control fumonisin contamination.