The triatomine bug Triatoma infestans was probably originated in Bolivia and dispersed passively over large areas of South America, where it is the major vector of Trypanosoma cruzi. In its probable origin area this species shows two different patterns of behaviour, being found both in sylvatic and human related habitats. Such behaviour is not observed in other areas of its distribution, where it is exclusive to human related habitats. The main objective of the present study was to evaluate head morphometry characters and isoenzymes patterns of two T. infestans populations, one collected in Minas Gerais (Brazil) and other in the Cochabamba Valley (Bolivia), in order to elucidate the factors that could have an implication with the different colonization behaviour. The two populations presented differences in both morphometric (head size) and isoenzymes (specially PGM enzyme profile). The insects from the colonies reared for several years in laboratory conditions, showed reduction in head size starting from the third generation, maintaining this same size in all generations after, until 10 years after generation. The laboratory rearing conditions could be similar to human associated habitats, inducing certain level of genetic homogeneity. Our results showed increase of genetic homogeneity in Brazilian and laboratory-reared colonies from Bolivia, that may be due to a selection process due to passive dispersion, followed by several founder effects episodes and genetic drift.