We report the cloning and sequencing of 18 mutant alleles of the benA, beta-tubulin gene of Aspergillus nidulans that confer resistance to the benzimidazole antifungal, antimicrotubule compounds benomyl, carbendazim, nocodazole, and thiabendazole. In 12 cases, amino acid 6 was changed from histidine to tyrosine or leucine. In four cases, amino acid 198 was changed from glutamic acid to aspartic acid, glutamine, or lysine. In two cases, amino acid 200 was altered from phenylalanine to tyrosine. These data, along with previous data indicating that amino acid 165 is involved in the binding of the R2 group of these compounds [Jung and Oakley, 1990: Cell Motil. Cytoskeleton 17:87-94], suggest that regions of beta-tubulin containing amino acids 6, 165, and 198-200 interact to form the binding site of benzimidazole antimicrotubule agents. These results also suggest that the presence of phenylalanine at amino acid 200 contributes to the great sensitivity of many fungi to benzimidazole antimicrotubule agents.