Squirrel monkeys (genus Saimiri) give birth to relatively large neonates with large, fast-growing brains. Maternal energy expenditure during gestation and infant development is argued to be high, but may be offset by the provisioning of offspring by females other than the mother (allonursing). Milk composition is an important component of maternal energy expenditure, but has been examined in only a small number of primate species. Here, we report on the milk composition from laboratory-housed Bolivian squirrel monkey (Saimiri boliviensis boliviensis) dams (n = 6) and allomothers (n = 2). Milk samples (n = 16) representing mid-lactation were assayed for fat, sugar, dry matter (DM), crude protein (CP), and fatty acids. Gross energy (GE) was calculated from these constituents (excepting fatty acids). The goals of this project were: (1) to provide descriptive data on milk composition of squirrel monkeys, including the range of intraspecific variation; (2) to determine if milk produced by allomothers differs from milk from dams; and (3) to compare squirrel monkey milk to that of other small New World monkeys, the callitrichines. Squirrel monkey samples averaged 4.56% fat, 3.59% CP, 6.98% sugar, 16.59% DM, and 0.91 kcal/g. The proportion of the medium chain fatty acids 8:0 and 10:0 was 40 times greater than that reported for human milk samples, and 18:1 and 18:2n-6 comprise more than 60% of total fatty acids. Milk from allomothers was lower than dams in fat, DM, and GE, which may relate to variation in maternal condition between these two groups. Excluding allomothers, milk from squirrel monkeys was higher in mean GE than captive common marmosets, but did not differ in the proportion of energy from fat, CP, and sugar relative to total GE. The consistency in energy from protein between species suggests this may be a shared-derived trait of New World monkeys. Am. J. Primatol. 70:35–43, 2008. © 2007 Wiley-Liss, Inc.