Cladosporin is a new fungal metabolite produced in good yield in the mycelium of Cladosporium cladosporioid s (Fres.) de Vries. Its structure was established as 3, 4-dihydro-6, 8-dihydroxy-3-(tetrahydro-6-methyl-2H-pyra -2- yl)methylisocoumarin on the basis of spectroscopic studies of the parent compound, mono-and di-methyl derivatives, and a monoacetate. Both cladosporin and monoacetyl cladosporin completely inhibited growth of severa dermatophytes on agar medium at a concentration of 75 μg/ml. The germination of spores from several Fenicillium and Aspergillus spp. was inhibited by these compounds at concentrations of 40 μg/ml or less in liquid medium.