Nut-cracking behavior in wild chimpanzees (Pan troglodytes verus) using natural stone hammers and anvils was studied through transect survey and experimental analysis at Bossu, Guinea. Transect surveys revealed that hammers were significantly smaller than anvils, although the role division was not strict. Analysis of experiments such as separating nuts and stones found under palm trees revealed the chimbanzees' flexibility in tool use. The behavioral patterns of transportation, discrimination, and the substitution of tools were not stereotyped. This flexibility appears to stem from the chimpanzees' understanding of the relationship between tools and referents, i.e. the function of tools.