The stable carbon and nitrogen isotope composition of bone collagen has been determined for 238 modern nonhuman mammals, comprising 43 species of herbivores and carnivores from grasslands and montane forests in Kenya and Tanzania, and 97 historic and prehistoric humans, representing 12 populations with different dietary adaptations from eastern and southern Africa. Among nonhuman species carbon isotope ratios separate grazers from browsers in open grasslands, forest floor from forest canopy, and forest floor from open grassland feeders. Water-dependent herbivores have lower nitrogen isotope ratios than drought-tolerant ones in the same habitat. Among human populations, carbon isotopes differentiate those who regularly consume grains and/or the protein of browsing animals from those who consume wild plants, nongrain crops, and/or the protein of wild animals. Nitrogen isotopes differentiate those dependent on the milk, meat, and blood of domestic animals or marine resources from those dependent mainly on plant foods. Combined use of carbon and nitrogen isotopes permits the differentiation of pastoralists from farmers, camel pastoralists from capri-bovine pastoralists, and grain farmers from nongrain farmers. The apparent presence of physiological and climatic influences on nitrogen isotope ratios within trophic levels complicates dietary interpretations of variations in nitrogen isotope ratios but may also offer a new tool of climatic reconstruction. It should nonetheless be possible to use the analysis of isotopic composition of bone collagen as a quantitative technique for the reconstruction of diet, habitat selection, climate, and water balance in prehistoric animal and human communities, and to test models that suggest dietary adaptations were prime movers of hominid biological and cultural evolution. -- AATA