Interaction of truffle mycelium with the host plant involves the excretion of extracellular enzymes. The ability of Tuber maculatum mycelium to produce an extracellular cellulase during submerged fermentation was demonstrated for the first time. T. maculatum mycelia were isolated and tested for extracellular cellulase production at variable pH on solid agar medium, and the highest activity was observed at pH 7.0. Furthermore, T. maculatum was subjected to submerged fermentation in basal salt medium for cellulase production. Under optimized conditions using sodium carboxymethyl cellulose (0.5 % w/v) as carbon source and an initial pH of 7.0, the enzyme production yielded 1.70 U/mL of cellulase in the cell-free supernatant after 7 days of incubation time. The optimum of the obtained cellulase's activity was at pH 5.0 and a temperature of 50 A degrees C. The enzyme showed good thermostability at 50 A degrees C by retaining 99 % of its maximal activity over an incubation time of 100 min. The cellulase activity was inhibited by Fe2+ and slightly activated by Mn2+ and Cu2+ at 1 mM concentration. The results indicated that truffle mycelium is utilizing cellulosic energy source in the root system, and the optimal conditions are those existing in the acidic Finnish soil.