In a house infested with P. megistus in an area endemic for Chagas' disease in the State of Bahia, Brazil, five of six examined specimens of the guinea pig Cavia porcellus were found naturally infected with T. cruzi. Cavia porcellus is not considered to be an epidemilogically important reservoir for T. cruzi in this locality. However its epidemiological importance should be considered in Brazil if people breed it in dwellings in endemic areas, as in Bolivia and Peru. Cavia porcellus is highly susceptible to infection with T. cruzi and may serve as an excellent source for infecting several especies of triatomine bugs.