During three field trips to the community of 'La Humedad', municipality of Santiago Jamiltepec, Oaxaca (a State in the southern part of the Mexican Republic), live specimens of Triatoma mazzottii Usinger (Hemiptera: Reduviidae) were found infected with Trypanosoma cruzi. The insects were captured in only one of the dwellings inspected in the locality, and in four small caves where bats and other mammals live. These findings suggest the need for further studies on the existence of a sylvatic cycle of Chagas' disease in Mexico, which has not yet been established.