We have previously shown that increased sensitivity to diethofencarb in the carbendazim(MBC)-resistant F914 strain of Neurospora crassa is caused by a single amino-acid change in beta-tubulin, 198Glu to Gly. Three diethofencarb-resistant mutants that are also resistant to MBC were isolated from strain F914. They contained single base-pair-substitution mutations in the beta-tubulin gene. The amino acid changes in beta-tubulin, Phe from 250Leu, Val from 165Ala, and Ala from 237Thr, were responsible for diethofencarb-resistance in the mutant strains FR511, FR513, and FR421, respectively. The amino acid at position 198 of beta-tubulin in these mutants was Gly, which is the same as in strain F914. beta-tubulin genes with 198Glu were constructed by site-directed mutagenesis. The altered beta-tubulin genes derived from FR511 and FR421 transformed the wild-type strain to resistance to MBC, indicating that 250Phe and 237Ala in beta-tubulin are responsible for resistance not only to diethofencarb but also to MBC.