Abstract The natural and stable isotopes of carbon vary in systematic ways largely determined by the photosynthetic pathway (C 3 or C 4) which fixes atmospheric CO 2 into organic matter. Within both C 3 and C 4 species there is a genetic component to the variation in this ratio which may be as great as 3‰. A variety of environmental factors also affect this ratio in plants. Individual biochemicals differ from each other in their isotopic values. The ratio in bulk plants is transferred to higher trophic levels with reasonable fidelity; however, the collagen-diet spacing is variable and needs to be understood. An appreciation of these sources of variation is necessary as we quantify our diet interpretations.