The Upper Paraná Atlantic Forest (UPAF) is one of the planet’s most endangered environments on account of its being highly fragmented, a process that is threatening the largest natural predators of this area, the jaguar (Panthera onca) and the puma (Puma concolor). The main objective of this dissertation was to investigate the impact of the UPAF fragmentation and land use change on the persistence of both felids. Analysis of satellite images were used to describe changes in land use. Agricultural expansion and the transformation of more than 75.000 km2 of native forest characterized the UPAF between 1973 and 2004, though the process of change was different in each of the three countries sharing this environment (Argentina, Brazil and Paraguay). A monitoring participatory network was created and volunteers were trained to collect samples from big felids, such as feces and tracks, throughout the UPAF. From these samples I derived presence data for these species from which I mapped their current distribution in the UPAF. Puma, jaguar and big canid tracks collected at zoos were used to evaluate traditional track identification methods which proved to show high error rates. Morphometric variables measured on zoo tracks were used in discriminant function analysis in order to improve differentiation between tracks of these species obtained by the participative network. Feces were identified by means of molecular techniques. Jaguar presence was confirmed in a large area of the Green Corridor in Misiones, and in the largest forest remnants of Brazil and Paraguay. Pumas showed a wider distribution, being recorded throughout Misiones province and including areas of Brazil and Paraguay where jaguars were not detected. An ecological niche factorial analysis showed that pumas and jaguars are mainly restricted to areas with native forest and avoid highly modified areas that are readily accessible to humans. The jaguar, however, proved to be a more sensitive species to landscape changes and anthropic pressures. Generalized lineal models indicate that jaguar persistence is determined not only by present conditions of this fragmented forest, but also by the fragmentation history of the landscape. Characteristics of surrounding land use and human pressure were also important to determinate jaguar presence in the forest remnants of the UPAF. A bidimensional model of habitat suitability allowed to determine the probable spatial structure of jaguar population and to detect priority areas to implement different management actions to preserve both this species and the UPAF.