The stilbene glucoside isorhapontin (5,4′-dihydroxy-3′-methoxystilbene-3-β-D-glucoside) is the major constituent of the ethyl acetate extracts fromPicea glehnii bark. Isorhapontin inhibited the hydrolytic activity ofTrichoderma cellobiohydrolase I (CBH I) for both bacterial microcrystalline cellulose and the soluble cellooligosaccharide celloheptaitol. The inhibitory effect for celloheptaitol, however, was more drastic than that for bacterial microcrystalline cellulose. The hydrolytic activity of the CBH I core domain for celloheptaitol was also inhibited by isorhapontin to a similar extent, suggesting that the interaction between isorhapontin and the core domain of CBH I is the reason for this phenomenon. The inhibition of CBH I activity by isorhapontin showed mixed noncompetitive and uncompetitive types in a concentration of the inhibitor of less than 125μM. TheKi andKi′ values were estimated to be 57.2 and 33.3μM, respectively. Whereas isorhapontin strongly inhibited CBH I activity, its aglycone isorhapontigenin (3′-methoxy-3,5,4′-trihydroxystilbene) showed almost no inhibition. Consequently, both the stilbenic and theβ-glucosidic structures in isorhapontin are essential for the inhibitory effect on CBH I activity. Isorhapontin also inhibited the activity ofTrichoderma endoglucanase I for celloheptaitol, whereas almost no effect was observed for the activities of both endoglucanases II and III.