We explored how the common vampire bat Desmodus rotundus is able to survive on hyper-arid rocky islands along the Peruvian coast in the absence of plants and terrestrial mammals. We used carbon and nitrogen stable isotope analyses on D. rotundus hairs and feces to assess the primary source of food, and compared our results with field observations near D. rotundus colonies. Data from stable isotope analyses were consistent with field observations, indicating that D. rotundus feeds on the South American sea lion Otaria flavescens, and occupies the third trophic level in a chain linking anchovies Engraulis ringens, O. flavescens, D. rotundus and a hippoboscid fly, Trichobius parasiticus. Our report demonstrates the importance of marine-derived resources for D. rotundus along the Peruvian coast. © Inter-Research 2008.