The widespread geographical distributions of mammalian carnivores such as the Carnivora and the Dasyuridae have often been erroneously equated with abundance. Their low densities and high demands on habitat area can render mammalian carnivores especially vulnerable to habitat fragmentation and destruction. The fossa Cryptoprocta ferox (Viverridae) is a mammalian carnivore threatened by the rapid loss of Madagascar's forests, to which it is endemic. A 3-year mark-recapture study, comprising four censuses, generated an estimate of fossa population density at 0.18 adults km−2, or 0.26 individuals km−2. This was supported by a similar estimate from home range data. The fossa is thought to be unusually common in the study area, yet the estimated density was lower than that predicted for a typical tropical carnivorous mammal of the body mass of a fossa. Ecologists are frequently under pressure to provide estimates of local and global population numbers of their study species; we discuss the numerous factors that limit our ability to do this on the basis of a single population estimate. Nonetheless, our findings are sufficient to suggest that none of Madagascar's 46 protected areas supports a viable population of fossas, indicating a demand for corridors and enlarged reserves to ensure this species' long-term survival. Loss of the top predator can have a knock-on effect on an ecosystem. The findings indicate that, to maintain intact tropical forest ecosystems, it may be essential to consider the requirements of their often little-known mammalian carnivores. These requirements could be far greater than previously assumed.