Allonursing, the behaviour of females nursing offspring that are not their own, is relatively frequent in capuchin monkeys. Using focal-animal sampling and ad libitum observations we describe the pattern of allonursing in a wild group of tufted capuchins, Cebus nigritus (4 cohorts, 22 infants), at Iguazú National Park, north-eastern Argentina, and test several hypotheses on the adaptive value of allonursing. During 2,351 contact hours with the group (including 4,207 focal-animal samples totalizing 329 h focused on infants) we observed 39 allonursing bouts. Infants were not allonursed more frequently by close kin than by more distant allomothers. Offspring of dominant females were allonursed more frequently than those of low-ranking females. Nursing bouts were longer than allonursing bouts. Our results suggest that allonursing in tufted capuchins has a social function and is not mainly aimed at providing milk to infants.