Mutants of 'N. crassa' highly resistant to benomyl with respect to hyphal growth were obtained by mutagenizing wild type strain St. Lawrence 74A or mutant Bml 51l(r) which is moderately resistant to the fungicide. One of these, strain El-91 that has a mutation mapping at the Bml (beta-tuhulin) locus, showed temporary sensitivity to the benzimidazole.