The action of high concentrations of benomyl was investigated in the wild-type strain St Lawrence of Neurospora crassa and in mutants displaying various levels of tolerance to the drug. Germ-tube formation by macroconidia, germination pattern (single versus multiple germ-tube emergence) and hyphal tip extension of these strains showed different sensitivities to the microtubule inhibitor, benomyl. The wild-type strain, whose vegetative growth was inhibited at micromolar concentrations, still germinated by multipolar outgrowth at 100 μm. At such a level, germination of the moderately resistant mutant Bml 511(r) was inhibited more than the wild type. Highly resistant mutants capable of hyphal growth at 500 μm were obtained by mutating strain Bml 511(r) using uv. One of these, strain E1-91, showed temporary sensitivity to the microtubule inhibitor and produced multiple germ-tubes. This strain was deficient in the β-tubulin polypeptide during the first hour of germination.