AbstractStable carbon isotope ratios in bone collagen and oxygen isotope ratios in bone and enamel phosphate from 25 individuals from the residential compound of Tlajinga 33 were used to examine the possibility that the inhabitants, who were craft producers, may have accepted immigrants to maintain either their ability to reproduce themselves as a social group or their level of economic productivity. Bone δ18O and δ13C values provide a long-term picture of geographic identity and diet, and enamel δ18O values provide a snapshot of geographic location during particular tooth development. A considerable proportion (29 percent) of the Tlajinga 33 inhabitants grew up elsewhere, but the majority of these immigrants had dwelt in Teotihuacan for many years before their death. Neither geographical relocation nor dietary differences are significantly associated with gender. The social position of foreigners appears to have been generally high. For example, the occupants of Tomb 50 appear to have come from elsewhere, possibly West Mexico, but foreigners were also found in lower status contexts such as middens. The stable isotope ratios reflecting long-term dwelling at Teotihuacan suggest that social status was achieved, which supports current archaeological evidence. Furthermore, the lack of dietary differences between immigrants and native Teotihuacanos may also imply political and/or ethnic assimilation.