Paxillus involutus (Batsch.: Fr.) Fr., an ectomycorrhizal fungus, is capable of forming sclerotia, resistant vegetative propagules. Sclerotia were induced in vitro on agar-solidified modified Melin Norkrans medium by exposure of the mycelium to temperatures between 6 and 14 °C. The number and size of sclerotia varied greatly with temperature. Mycelium exposed to either 6 or 8 °C produced many smaller sclerotia (less than 1.0 mm in diameter) lacking droplets of exudate, whereas mycelium exposed to temperatures between 10 and 14 °C formed fewer, larger sclerotia (1.0 mm or more in diameter) with abundant exudate. Sclerotia were not induced from mycelium grown in liquid medium at 10 or 25 °C or from mycelium on agar-solidified medium maintained at 25 °C under a variety of conditions, including changes in medium composition and volume. Key words: sclerotia, Paxillus involutus, temperature, ectomycorrhiza, in vitro.