We describe the diet of fossas (Cryptoprocta ferox) in a dry deciduous forest of western Madagascar from 376 scats collected between June 1994 and September 1996, from which 554 prey items were identified. More than 90% of these were vertebrates, and more than 50% were lemurs. No other nonprimate mammal includes such a high proportion of primate items in its diet. The principal prey comprised approximately 6 lemur species and 2 or 3 spiny tenrec species, along with snakes and small mammals. Significant differences were apparent in the composition of the scats in wet and dry seasons, with a higher proportion of Tenrec in the former, and fewer lemurs. Within the confines of a diet of vertebrates, fossas appeared to be opportunistic predators. For those prey types for which data were available, a significant relationship was found between the estimated relative number of individuals taken of any one type of prey and its abundance. Fossas were estimated to remove up to 19% of their prey populations per year. This high impact suggests that they were living close to the maximum population density possible on the available prey. Species of a wide range of body masses were included in the diet. Verreaux's sifaka (Propithecus verreauxi), weighing more than one-half of the body mass of the fossa, constituted approximately 11% of the prey biomass.