Solari, A., Wallace, A., Ortiz, S., Venegas, J., and Sanchez, G. Biological characterization ofTrypanosoma cruzistocks from Chilean insect vectors.Experimental Parasitology89,312–322. Fifty-sevenTrypanosoma cruzistocks isolated fromTriatoma infestansandTriatoma spinolaiof the five different geographic endemic areas of Chile were studied by schizodeme and molecular karyotype analysis. Four different genotypes are found in the sylvaticT. spinolaivector and five in theT. infestansdomiciliary vector. Of these genotypes, two common genotypes overlap on both transmission cycles exclusively in the extreme northern endemic areas of Chile. Metacyclic trypomastigotes obtainedin vitroor cell-derived trypomastigotes proved to be infective in γ-irradiated Balb/c mice for the study of the immune response and biological behavior. Of a total of 57T. cruzistocks obtained, 19 of them, representing all the different genotypes found in Chile, were tested on a murine experimental model and then fully studied. Female compared with male animals demonstrated greater resistance to Chagas disease with all theT. cruzistocks tested. The immune response was assessed by lytic antibodies that were studied by thein vitroantibody-dependent complement-mediated lytic assay with the use of bloodstream trypomastigotes as target cells. In one unique parasite genotype the elicited lytic antibodies reacted in a genotype-specific manner, in contrast with lytic antibodies generated by otherT. cruzigenotypes. Parasitemias were high, moderate, and low, with mortality ranges of 6–50%, 0–45%, and 0–10%, respectively. No association was found between specific infective genotypes and virulence or mortality. Independently of theT. cruzistrain studied, each population displayed a characteristic parasitemia curve and prepatent period. A considerable number of the parasite stocks proved to be mixed populations, according to molecular karyotype patterns obtained before and after differentiation and amplification of the parasites. This fact created difficulty in assessing the identity of the genotype really infective to mice.