We examined patterns of infant care and the relative importance of female age, rank and relatedness to allomaternal care for a wild population of wedge-capped capuchin monkeys, Cebus olivaceus in Venezuela. Mothers interacted primarily with their own infants throughout the study. Infant age affected the timing and type of allomaternal interaction; investigation occurred primarily in the first 3 months of life, carrying was dominant during the second 3 months, and association occurred primarily in the third 3 months. The onset and level of allomaternal care assured a high level of infant care as maternal care was declining, suggesting that allomaternal care is an important component of infant survival. The relative importance of female age, rank and relatedness varied for different behaviour. Relatedness was the most consistently important effect. Sibling females participated in allomaternal care almost 4 times more often than other females and were especially active in carrying and associating with infants. Female rank was an important effect when the coefficient of relatedness was less than 0.5: high-ranking females participated in allomaternal care more than low-ranking females. Old juveniles and nulliparous adults interacted with infants more than young juveniles and parous adults, but the effect of female age was relatively unimportant. Allomaternal nursing may represent a form of reciprocal altruism. Allomaternal nursing supplemented maternal nursing and was unrelated to kinship or rank of female.