We examined the host selection of common vampire bats (Desmodus rotundus) with stable carbon isotopes in an area that offered 2 isotopically contrasting food sources, either introduced livestock living on C₄ plants or native rainforest mammals living on C₃ plants. We predicted that vampire bats would have carbon isotope ratios (δ¹³C) similar to those of livestock if they select exclusively cattle. The δ¹³C of vampire bats averaged −10.3 ± 2.6‰ (mean ± SD) and was almost identical to that of livestock (X̄ = −12.9 ± 1.6‰). Thus, both vampire bats and livestock are clearly members of the C₄ food web. Typical rainforest mammals such as the fruit-eating bat Carollia perspicillata had a δ¹³C of −24.4 ± 0.6‰, which identified them as members of the C₃ food web. The stable carbon isotope signature of local vampire bats implies a high degree of preference for cattle. We suggest that the population expansion of D. rotundus is only indirectly linked to increasing host densities and is more directly related to the bats' preference for livestock over native mammals, probably because fenced-in cattle are a more predictable resource than free-ranging natural hosts.