In the course of our screening of beta-glucosidase inhibitor, a culture filtrate of a mushroom, Phellinus sp. strongly inhibited the enzyme activity. The active substance was isolated through charcoal separation, column chromatography and crystallization. Spectroscopic and crystallographic analysis revealed that it had a novel cyclitol structure, (1S,2R,3S,4R,5R,6R)-5-hydroxymethyl-7-oxabicyclo[4,1,0]heptane-2,3,4-tri ol, and we named it cyclophellitol. It inhibited almond-derived beta-glucosidase with an IC50 of 0.8 micrograms/ml.