A population viability analysis (PVA) was carried out for populations of the arboreal didelphid Micoureus demerarae in eight small (1.5–15.0 ha) forest fragments in south-eastern Brazil. Analysis was based on field data obtained through demographic studies carried out since 1995. Populations are small, but connected by dispersing individuals, thus forming a metapopulation. Frequency of catastrophic fires was estimated from the Reserve's historical records. We used the computer package VORTEX for all analyses. All populations and the metapopulation were found to be endangered within 100 years (extinction probability > 0.98). A sensitivity analysis was run varying six parameters: three demographic (sex ratio, migration and mortality rates), two environmental (K, fire frequency) and one genetic (level of inbreeding depression). Genetics, K, mortality rates and sex ratio seemed to play major roles to population persistence, whereas catastrophes and migration rates had a secondary role. Among demographic factors, extinction rate was least sensitive to migration rate. Micoureus demerarae can be used as a model species, thus improving our knowledge of how extinction-prone populations of neotropical arboreal marsupials in forest fragments might be, and which management actions could decrease such risks.