The formation of pentachloroanisole by methylation from pentachlorophenol by Trichoderma virgatum in liquid cultures was detected using gas chromatography and confirmed by melting point determination and infrared (i.r.) spectroscopy. The formation of pentachloroanisole, however, did not account for the total loss of pentachlorophenol in the medium, suggesting that this reaction is only the first step in the metabolism of this compound or a parallel reaction to degradation. Pentachloroanisole was much less toxic than pentachlorophenol to Trichoderma virgatum, Cephaloascus fragrans, and Penicillium sp., as well as to fish in laboratory toxicity tests.