A large set of delta¹³C values for both the organic (collagen) and inorganic (apatitic) components of bone and tooth is presented for southern African fauna. Equations are obtained to describe variation in the relationship between these two values (the apatite-collagen spacing) with trophic level for herbivores, carnivores, and omnivores. The empirically derived equations are slightly different from earlier predictions, and the implications are discussed. Differences between apatite and collagen delta¹³C values in a sample of prehistoric human skeletons from the southwestern Cape coast are considered in the light of relationships derived from faunal data. Unexpectedly small isotopic differences seen in many of these skeletons may be attributable to the particular mix of marine and terrestrial components in their diets.