Neurospora crassa osmosensitive (os) mutants are sensitive to high osmolarity and therefore are unable to grow on medium containing 4% NaCl. We found that os-2 and os-5 mutants were resistant to the phenylpyrrole fungicides fludioxonil and fenpiclonil. To understand the relationship between osmoregulation and fungicide resistance, we cloned the os-2 gene by using sib selection. os-2 encodes a putative mitogen-activated protein (MAP) kinase homologous to HOG1 and can complement the osmosensitive phenotype of a Saccharomyces cerevisiae hog1 mutant. We sequenced three os-2 alleles and found that all of them were null with either frameshift or nonsense point mutations. An os-2 gene replacement mutant also was generated and was sensitive to high osmolarity and resistant to phenylpyrrole fungicides. Conversely, os-2 mutants transformed with the wild-type os-2 gene could grow on media containing 4% NaCl and were sensitive to phenylpyrrole fungicides. Fludioxonil stimulated intracellular glycerol accumulation in wild-type strains but not in os-2 mutants. Fludioxonil also caused wild-type conidia and hyphal cells to swell and burst. These results suggest that the hyperosmotic stress response pathway of N. crassa is the target of phenylpyrrole fungicides and that fungicidal effects may result from a hyperactive os-2 MAP kinase pathway.Wild-type Neurospora crassa strains can grow on media with different osmotic strengths. The members of one class of N. crassa mutants, known as os (osmosensitive) mutants, however, are sensitive to hyperosmotic pressure and are unable to grow on media supplemented with 4% NaCl (wt/vol) or 1 M sorbitol (25). Several os mutants, including os-1, os-2, os-3, os-4, os-5, os-6, and cut mutants, and the sorbose-resistant sor (T9) mutant have been described (25). Most os mutants have aberrant colony morphology on regular Vogel's medium N and form sticky, close-cropped aerial hyphae. The aggregated hyphae are intensely pigmented and have a tendency to rupture and bleed (25). In addition, os mutants have reduced conidiation and altered cell wall compositions (10, 16).