No significant differences were found between the fungitoxicities of a commercial pentachlorophenol and one in which the impurities (hexaand octachlorodibenzo-p-dioxins) commonly found in commercial pentachlorophenols were markedly reduced. The brown rots had lower tolerences to pentachlorophenol than did the white rots. One isolate of the white rot fungus <i>Polyporus tulipiferae (Madison 517) showed a high tolerance to pentachlorophenol and might be considered as a standard assay organism for this chemical. (A product with reduced impurities might cause less staining.) -- AATA