Prevention of vector-borne transmission of Chagas disease mainly relies on residual insecticide spraying. Despite significant success at a regional scale, house infestation with Triatoma infestans (Klug) (Hemiptera: Reduviidae) still persists in the Gran Chaco ecoregion. One key aspect is the identification of the sources of reinfestant triatomines. After detecting fine-scale genetic structure in two rural villages of Pampa del Indio, Argentine Chaco, we tested hypotheses on the putative origins of the triatomines collected at 4, 8, and 12 mo after insecticide house spraying. We genotyped 10 microsatellite loci in 262 baseline and 83 postspraying triatomines from different houses. Genetic variability was similar between baseline and postspraying populations, but 13 low-frequency alleles were not detected at postspraying. F_STs were not significant between insects collected before and after insecticide spraying at the same house in all but one case, and they clustered together in a neighbor-joining tree. A clustering algorithm detected seven genetic groups, four of them mainly composed of baseline and postspraying insects from the same house. Assignment tests suggested multiple putative sources (including the house of collection) for most postspraying insects but excluded a house located more than 9 km from the study area. The origin of three triatomines was attributed to immigration from other unaccounted sources. Our study is compatible with the hypothesis that house reinfestations in the Argentine Chaco are mostly related to residual foci (i.e., survival of insects within the same community), in agreement with field observations, spatial analysis, and morphometric studies previously published.