An altered beta-tubulin gene that confers resistance to benomyl [whose active ingredient is 2-(methoxycarbonylamino)benzimidazole (MBC)] was isolated from a DNA library of Aspergillus flavus and used as a selectable marker for transformation. The beta-tubulin gene was cloned into a plasmid vector containing the pyr-4 gene of Neurospora crassa, and transformants were selected either for uracil prototrophy or MBC resistance. Transformants selected for uracil prototrophy were of three phenotypic classes: sensitive, intermediate, and resistant to MBC. Transforming DNA appeared to integrate at several sites in the genome, with the more resistant phenotypes having more copies of the altered beta-tubulin gene than the sensitive and intermediate phenotypes. Transformants were also selected on medium containing MBC. The average frequency of transformation (1 to 3 transformants per micrograms of transforming DNA) was lower than that obtained by selection for uracil prototrophy, presumably because of failure to select transformants that contained few copies of the altered beta-tubulin gene. The sequence of the beta-tubulin gene was determined and compared with the published sequence of the benA gene of A. nidulans; the beta-tubulin gene was found to be highly conserved between the two Aspergillus species. Notable differences were that the beta-tubulin gene of A. flavus lacks intron 6 present in benA and has an additional leucine at position 148. This is the first gene sequence reported from an aflatoxin-producing fungus and adds to the growing body of knowledge of the beta-tubulin genes and their use as selectable markers for transformation of filamentous fungi.