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Distinguishing between sheep and goats in C4 grass environments using new dental morphology criteria and enamel bioapatite stable carbon isotope ratios (?13C) was tested on 35 modern individuals from the Central Rift Valley of Kenya. Two morphological criteria on the second and third lower molars, one of which had been previously partially described by Halstead et al. [Journal of Archaeological Science 29 (2002) 545], were found to be highly reliable in this population. Identification of species using carbon isotope ratios is made possible in some circumstances by differences in the feeding behavior of sheep, which are mainly grazers, and goats, which are mainly browsers. In environments where C4 grasses predominate, sheep include a higher proportion of C4 plants in their diet, and thus have higher ?13C values than goats. In the present study, the annual range and seasonal variation of carbon isotope ratio of diet of sheep and goats was measured from intra-tooth sequential analysis. Although the ranges of goat and sheep ?13C values overlap, those higher than -3.4[per mille sign] all belong to sheep; values lower than -5.2[per mille sign] all belong to goats. There is no overlap of the mean ?13C values by tooth, which range from -11.8[per mille sign] to -4.2[per mille sign] for goats, and from -3.1[per mille sign] to -1.3[per mille sign] for sheep. These results suggest that carbon isotope analysis of bone collagen and/or apatite will also distinguish sheep from goats in tropical C4 grasslands. Application of the ?13C criteria to archaeological material must be restricted to C4-dominated environments, and where potential access to C3 plants (mobility, foddering) can be assessed. The utility of these morphological and isotopic criteria for differentiating sheep and goat breeds in other regions remains to be evaluated.