Stable isotope analysis of bone collagen is now a well-established method of studying ancient human diet. Carbon isotope values distinguish between C-3 and C-4 plants in the terrestrial food web; nitrogen values can indicate marine resource exploitation, terrestrial climate, and trophic level. Unfortunately, the relative contributions of the protein, carbohydrate, and fat portions of the diet to bone collagen and bone apatite are still not fully understood. Stable isotope data for human burials from the Preclassic Maya site of Cuello, Belize demonstrate that isotopic analysis of both tissues is necessary for proper dietary reconstruction of all but the simplest ancient food webs. Equally important are isotopic analyses of the fauna and nora available for human exploitation, and the integration of these data with archaeological evidence. At Cuello, it appears that maize-eating dogs may have been a significant dietary component, but there is no evidence that deer were tamed or loose-herded as ethnohistoric accounts suggest.