Matjes River Rock Shelter is a large shell midden on the southern coast of South Africa. Stable nitrogen (δ15N) and carbon (δ13C) isotope ratios were measured in bone collagen and dentine from human skeletons excavated from this site in order to establish a weaning curve in mid-Holocene hunter-gatherers. δ15N results show a progressive increase in individuals from birth to 1.5 years old. δ13C results are more tightly clustered and mirror the steady progressive change seen for δ15N. We deduce that children at Matjes River Rock Shelter were breastfed for at least the first 1.5 years after birth, and were weaned sometime between 2–4 years of age. A similar pattern was documented for historic-era Kalahari foraging people, where the interbirth spacing was approximately 3 years. This study provides the first direct evidence for an extended period of breastfeeding, and thus long interbirth intervals, among prehistoric foragers, even when those foragers lived in an environment with abundant food resources. Am J Phys Anthropol, 2006. © 2005 Wiley-Liss, Inc.