After pointing out that American trypanosomiasis is an amphixenose which fits very well the Pavlovsky's doctrine of diseases with natural foci, the Author analyses the problem of multiplicity and diversity of such foci, which are due to the great variety of natural mammalian hosts and triatomine vectors. Some of the most common and important types of natural foci observed in the Northeastern Region of the State of São Paulo and nearby localities of the State of Minas Gerais are described; such foci are represented chiefly by soil and rock cavities and cracks, tree holes, anfractuosities and buttresses, bushes, century plants, and palm-tree tops, where T. cruzi circulates among wild mammals and triatomines.