Using comparable techniques we studied the abundance, habitat use, and diet of Darwin's fox (Pseudalopex fulvipes), as well as prey availability in two constrasting settings of southern Chile: on the mainland in the Nahuelbuta ranges, and in Chiloe Island. This fox lives in sympatry with its congener P. griseus in Nahuelbuta, but is the sole fox on Chiloe Island. We estimated that Darwin's fox is about twice as abundant in Chiloe than on the mainland. The structure of the habitat where Darwin's fox was found was remarkably similar between the two study sites despite floristic differences. Its diet, however, was markedly dissimilar, with mainland foxes preying extensively on mammals and reptiles and little on insects, and island foxes preying primarily on insects and amphibians, and little on mammals. The low consumption of mammals in Chiloe may be attributed to the low densities reached by that prey on the island as compared to Nahuelbuta (estimated at about double the insular density). Fruit consumption in Chiloe was almost three times higher than in Nahuelbuta, suggesting a lower energy diet for insular foxes. We also report behavioural observations on Darwin's fox provided by knowledgeable sources.