AbstractReports on use of stones as hammers and anvils to open hard nuts by wild capuchin monkeys are scarce and limited toCebus libidinosus. Here, we report for the first time data on tool use—stones as hammer and anvils to open nuts—in wildC. xanthosternosand a description of new tool using sites forC. libidinosus. Our records were made by visiting anvil sites and by information obtained from local residents. We surveyed three different biomes:Caatinga(dry forest and thorn scrub),Cerrado(Brazilian bush savannah), and Atlantic forest (wet forest), all records of tool use were fromCaatingaor transitional areas between habitats. The behavior is suggested to be routinely performed and widespread among several populations. The fruits of six plant species in different localities were opened with hammer stones byC. xanthosternos.Hammer stones were of similar weigh as those described in other studies ofC. libidinosus. Conditions found inCaatinga, such as a more frequent use of the ground by the monkeys and/or food scarcity, may play an important role in the acquisition of nut‐cracking behavior. The absence of more reports of nut cracking and other forms of tool use in other species of wildCebusis likely to result from a lack of surveys in very dry and food limited habitats or intrinsic characteristics of otherCebusspecies. Am. J. Primatol. 71:366–372, 2009. © 2009 Wiley‐Liss, Inc.