BACKGROUND: The geographical distribution of the Chilean flamingo (Phoenicopterus chilensis) includes the southern-central Neotropics. Despite its wide distribution, currently there is no dietary information on its southern distribution range. From June to September 2011, we quantified the diet and prey availability of the Chilean flamingo in the marine wetland of Caulín (41°48' S, 73°37' W), southern Chile. RESULTS: The prey availability related to both plankton and benthos were four species of copepods, four polychaetes, one foraminifera, and two amphipods. The diet of the Chilean flamingo was composed of foraminifera (Ammonia beccarii), copepods (Harpacticus sp.) and polychaetes. The most abundant prey items from feces of flamingos were Ammonia beccarii and Harpacticus sp. The diameter of A. beccarii consumed by flamingos ranged between 400 and 900 µm, while its width varied between 100 and 300 µm. The width of Harpacticus sp. consumed ranged between 160 and 260 µm. The similarity between flamingo diet and prey availability was 0.553. The diversity of prey organisms in the benthos was higher than that observed from plankton and feces of birds. A. beccarii was preferred over other prey consumed by flamingos. This preference is not related to the size of Harpacticus sp. but to their ability to swim and escape from flamingos. CONCLUSIONS: The dietary similarity of the Chilean flamingo versus the availability of prey in the environment (plankton and benthos) was 55%, indicating that the Chilean flamingo is a partially selective predator.