(Pless et al. 1992, Wincker et al. 1997, Brenière et al. 2002.Control of the parasite is hampered by the complex ecology of its transmission. T. cruzi has multiple mammalian hosts and insect vectors. Non-human mammal hosts include dogs, domestic and wild guinea pigs and various peri-domestic animals such as goats, sheep, and pigs. Vector populations, with the potential for transmission, can be found in domestic environments, where they feed on dogs and guinea pigs kept in homes, in peri-domestic homestead associations, and in sylvatic zones associated with wild guinea pigs. In Argentina, Brazil, Chile, and Uruguay, T. infestans is exclusively domestic or peri-domestic, thus eradication of the vector in these habitats, followed by vigilance against re-infestation, has proven largely successful in reducing transmission of T. cruzi and thus the prevalence of Chagas disease. In contrast, in Bolivia the vectors occur in domestic, peri-domestic, and sylvatic environments. Thus, control of T. infestans in towns and homesteads is confounded by the possible re-infestation from surrounding sylvatic areas. In addition, recent work in peridomestic sites in Argentiana has shown that spraying with pyrethroids does not always eradicate T. infestans and that infestations present a year after spraying resulted from individuals that had survived treatment (Gürtler et al. 2003). Audino et al. (2004) have shown that higher activities of degradative esterases and P450 contribute to deltamethrin resistance.It has been suggested that the Cochabamba valley, to the north of Chuquisaca, is the geographic origin and epicenter of spread of T. infestans (Torrico 1946, Schofield 1988. However, Dujardin et al. (1998) sampled from the Sucre region of Chuquisaca using allozymes, and in a comparison with other Andean and non-Andean regions determined that the Sucre-Vallegrande-Potosi area is the center of origin and dispersion of T. infestans. We conducted a study over a larger area within Chuquisaca using a fragment of the mitochondrial cytochrome b gene, as in Monteiro et al. (1999), for 62 specimens from periurban and rural localities in order to ascertain the population structure of T. infestans in this region and determine whether cyt b is a useful marker for this purpose.Evolutionary theory predicts higher genetic variation in ancestral sylvatic populations than in recently derived domestic populations, and in domestic populations that intermix with sylvatic populations (Schofield & Dias 1999). If the Cochabamba Valley, the neighboring Department of Chuquisaca or both are the geographic origin and epicenter of spread of T. infestans, insects collected in these regions should display more genetic variation. On the other hand, populations of T. infestans, which are highly adapted to domestic environment may show less genetic variation as a result of human dissemination.