Fumonisins are mycotoxins, produced mainly by Fusarium verticillioides, that are potentially carcinogenic to humans and toxic to animals. Synthesis of these toxins is directed by a cluster of 15 genes, among which FUM1 is the largest; it encodes a polyketide synthase. This enzyme probably catalyzes the synthesis of a polyketide that forms a large portion of the fumonisin structure. In this study, 27 strains possessing the FUM1 gene, as determined by polymerase chain reaction, were analyzed. A portion of the FUM1 gene was amplified and sequenced from 6 of 27 Brazilian strains isolated from corn and sorghum. The sequence similarity for the six F. verticillioides strains was almost 100%.