AbstractWe conducted an exploratory investigation in an area where nut‐cracking by wild capuchin monkeys is common knowledge among local residents. In addition to observing male and female capuchin monkeys using stones to pound open nuts on stone “anvils,” we surveyed the surrounding area and found physical evidence that monkeys cracked nuts on rock outcrops, boulders, and logs (collectively termed anvils). Anvils, which were identified by numerous shallow depressions on the upper surface, the presence of palm shells and debris, and the presence of loose stones of an appropriate size to pound nuts, were present even on the tops of mesas. The stones used to crack nuts can weigh >1 kg, and are remarkably heavy for monkeys that weigh <4 kg. The abundance of shell remains and depressions in the anvil surface at numerous anvil sites indicate that nut‐cracking activity is common and long‐enduring. Many of the stones found on anvils (presumably used to pound nuts) are river pebbles that are not present in the local area we surveyed (except on or near the anvils); therefore, we surmise that they were transported to the anvil sites. Ecologically and behaviorally, nut‐cracking by capuchins appears to have strong parallels to nut‐cracking by wild chimpanzees. The presence of abundant anvil sites, limited alternative food resources, abundance of palms, and the habit of the palms in this region to produce fruit at ground level all likely contribute to the monkeys' routine exploitation of palm nuts via cracking them with stones. This discovery provides a new reference point for discussions regarding the evolution of tool use and material culture in primates. Routine tool use to exploit keystone food resources is not restricted to living great apes and ancestral hominids. Am. J. Primatol. 64:359–366, 2004. © 2004 Wiley‐Liss, Inc.