We used stable carbon (δ13 C) and nitrogen (δ15 N) isotope analysis to investigate the trophic ecology of 8 small cetacean species of the southwestern South Atlantic Ocean: 6 delphinids (Grampus griseus, Lagenorhynchus cruciger, L. australis, Lissodelphis peronii, Pseudorca crassidens, and Cephalorhynchus commersonii) and 2 phocoenids (Phocoena dioptrica and P. spinipinnis). We also analyzed samples of possible prey collected from oceanic and coastal habitats adjacent to Tierra del Fuego. Cetacean bone-collagen δ13 C and δ15 N data revealed information on both habitat and prey preferences. We observed an isotopic continuum in which coastal species had the highest values of δ13 C and δ15 N (L. australis), while oceanic and southern species had the lowest values (L. cruciger and P. dioptrica), indicative of offshore foraging in cold oceanic waters near the Antarctic Convergence. Overlap in mean isotope values between C. commersonii and P. spinipinnis suggests that these species may have similar habitat and/or prey preferences. Isotope results for L. peronii, P. crassidens, and G. griseus suggest that at these latitudes (~54°S) they forage on the outer continental shelf. G. griseus show bimodal isotopic patterns, suggesting that 2 ecotypes that forage in different habitats and/or consume different prey items occur in this region of the southwestern Atlantic Ocean. The isotopic data presented here provide insight into the ecology of these cetaceans, with relevant implications for their successful management and conservation.