Capuchins (Cebus libidinosus) occupy areas of Caatinga in northeast Brazil. They consume the nuts of several species of difficult-to-open fruits (two species of Palmae and one species of Euphorbiacea) and are reported to use stones as hammers to crack open the nuts. This article describes the weight of hammers found on anvils and presumably used for nut-cracking by individuals in two groups of wild unprovisioned capuchin monkeys. Hammer weights ranged from less than 200 to over 3 kg. Based on a correlation between the type of broken nuts found at a site and the stones present on anvils, there was evidence that hammer weight differed according to nut size. These findings are consistent with experimental data recently published by Visalberghi et al. [Current Biology 19, 2009, DOI: 10.1016/j.cub.2008.11.064] and indicate that capuchins are capable of choosing stones of appropriate weight to effectively use pounding tools in natural environments without interference from humans. Am. J. Primatol. 72:270–275, 2010. © 2009 Wiley-Liss, Inc.