The behavioral ecology of the critically endangered Darwin's fox Pseudalopex fulvipes was examined during the reproductive season of 2001/2002 on a coastal population, on Chiloé Island in southern Chile. Foxes were radio-tracked and their diet and feeding behavior, activity patterns, home range sizes and spatial organization, habitat use and selection, social organization and abundance were studied. Foxes were solitary hunters and showed a generalist and opportunistic feeding behavior. Insects were the most abundant prey, followed by crustaceans, rodents, birds, amphibians, ungulates, reptiles, and marsupials. As marine organisms were frequently eaten, the ocean subsidy was important. Plant seeds were dispersed up to +650 m from their sources. Prey were hunted in all habitat types recognized, throughout the daily cycle, and were consumed as they were available along the season. All foxes were active throughout the day, but more so at night. Morphologically, aside from males having broader muzzles, they did not show external sexual dimorphism or differences in weight. The tail and feet of Darwin's foxes were relatively shorter than in other congeneric species. Individual home ranges and core areas ranged from 103 to 488 ha and from 30 to 130 ha, respectively, were similar between males and females, and larger than expected for foxes of that size. All home ranges were elongated (2930 m on average) following the shoreline. Foxes overlapped their home ranges extensively – and core areas less so – with individuals of the same or different gender, showing no apparent territorial behavior. They appear to be monogamous, allowing subordinates in their home ranges. The ecological density was 0.92 foxes km−2, which may be higher than in inland populations. Old-growth forest was consistently avoided by all individuals, second-growth forest and shrubland were used as available, and the use of dunes and other lands (mainly shores) was mixed.