Following reports of an unusually high incidence of acute Chagas's disease and the appearance of large numbers of Triatoma infestans in the southwestern region of the State of Bahia, triatomine bugs (Hemiptera: Reduviidae) and domestic animals in one of the affected communities were surveyed and examined for infection with Trypanosoma cruzi. Triatoma infestans was prevalent in houses and was also found in peridomestic habitats. T. sordida and T. pseudomaculata occupied peridomestic and sylvatic habitats and T. brasiliensis was found only among rocks far from houses. Panstrongylus megistus, formerly present in the region, was not found. Trypanosoma cruzi was detected in 19.5% of Triatoma infestans, 11.5% of T. sordida, 19% of dogs, 29% of cats and 100% of rats examined. A disproportionate number of early instar bugs were infected with Trypanosoma cruzi, suggesting that a rapid increase in the rate of transmission had recently occurred. The history of the domestic triatomine fauna of the region since 1912 is reviewed, and it is proposed that the relatively recent arrival of Triatoma infestans initiated a domestic cycle linked to peridomestic and sylvatic cycles of Trypanosoma cruzi transmission. Increased human mobility, the use of DDT for malaria control, and drought conditions are considered as factors which might have contributed to the outbreak of human infection.