A natural focus of American Trypanosomiasis was found in the city of S. Paulo. It was localized in a forest area preserved for residential purposes. In this focus the participation of triatomids bugs Panstrongylus megistus, marsupials of Diaelphis marsupialis species and domestics rats Rattus norvegicus was observed. Natural infection of all of them was observed. The ecotopes were found mainly in the pinus trees Cryptomeria japonica. Beside this, an adult female of P. megistus was found suckling actively humans inside the house. Some considerations are made in this paper concerning the distribution area of that species of triatomid. The sylvatic population found currently in South Brazil, represents probably the action of environmental factors limiting the bugs to its natural ecotopes. Nevertheless, these findings suggest that, from the epidemiological point of view, some approximation of the infection to the human environment was performed, at least, in urban areas.