With the increasing use of carbon and nitrogen stable isotope variations in studies of prosimian diets, it is essential to understand the full extent of this variation in food plants. Our goal here is to characterize the δ 13C and δ 15N values of plants and modern and subfossil lemurs in and near Beza Mahafaly Special Reserve, south-western Madagascar. The region is characterized by a long dry season and a shorter monsoon season. In general, we found a high degree of variability both intra- and interspecifically. Furthermore, some of the general assumptions of isotopic ecology were not supported by our results. We suggest that researchers should be cautious when interpreting isotope values, especially when the conclusions are used to model paleocommunities or to inform conservation policies.