6-Nitro-iso-vanillic acid, a new natural product, has been isolated from the fruit bodies of an Australian toadstool belonging to Cortinarius (Sericeocybe). Nitro compounds are found only occasionally as natural products, a small number having been isolated from higher plants, bacteria and fungi [1,2]. Only three nitroaromatic metabolites have been reported hitherto from Basidiomycetes; p-nitrotoluene (1) and gurunitrobenzaldehyde (2) occur in low concentrations in cultures of Lepista diemii [3], and 3,5,6-trichloro-1,4-dimethoxy-2-nitrobenzene (3) has been isolated from the fruit bodies of Fomes robiniae (Aphyllophorales) [4]. Of these, only the latter may be regarded as a contributor to the colour of the fungus in which it is produced. We report here the isolation and characterization of the first nitroaromatic pigment, namely 6-nitro-iso-vanillic acid (4), from a toadstool belonging to the order Agaricales.