The natural foci of the American trypanosomiasis, as all biogeocenoses, may remain relatively unchanged, this stability being a condition necessary to the existence of such foci. But the ecological equilíbrium may be temporarily or definitively modified; most important are the modifications due to direct or indirect man interference. Among the consequence of this interference the following possibilities may be pointed out: 1. reduction or disappearence of natural foci; 2. concentration of hosts and vectors in limited favorable areas; 3. displacement of host and vectors to new areas; 4. invasion of human dwellings and their annexes by wild hosts and vectors and the establishment of the domestic cycle of the infection. These possibilities are fully discussed and illustrated with appropriate examples.