The initial stage of cellulose degradation has been studied via in vitro assays of fungi isolated from rotten wood in a boreal forest. Among the 37 isolates, Antrodia sinuosa appeared to be an effective cellulose degrader and was selected for studying the initial degradation process. In the liquid cultivation with carboxymethylcellulose (CMC), the increase of the mycelial dry weight coincided with the pH decrease of the culture medium from pH 5.7 to 3.9, between the 3 ^rd and 6 ^th cultivation day. At the same time, the cellulolytic activity increased; the CMCase activity increased sharply and the reducing sugars reached their maximum concentration in the culture medium. It seems that the decreasing pH enables the cellulose degradation by A. sinuosa at an early stage of the process. The results of this study may be useful for a more efficient industrial application of biomass by means of brown-rot fungi.