Trypanosoma cruzi prevalence in triatomines and risk factors associated to the presence of the insect were studied in 990 rural houses in the southern region of the State of Mexico, Mexico. In each house, triatomines were collected, and information related to house construction material was obtained. T. cruzi infection was diagnosed in all triatomines. A primary screening was performed using 2 x 2 contingency tables of exposure variables. All variables with P <or= 0.20 were analyzed by logistic regression. Triatomines (N = 125) were collected from 822 houses and analyzed for T. cruzi infection. Triatoma pallidipennis (97.4%) and Triatoma dimidiata (2.6%) were identified in 52.1% of the localities and in 6.1% of the houses. Infection was found in 28.0% of triatomines, from which 28.9% were nymphs. Factors associated with triatomine infestation were flooring construction material (dirt floor: odds ratio [OR], 10.05; 95% confidence interval [CI], 5.31-18.04; P = 0.0001), house rooms (at least three rooms: OR, 2.04; 95% CI, 1.07-3.86; P = 0.028), and ceiling construction material (cardboard lamina tile: OR, 6.84; 95% CI, 1.49-31.31; P = 0.013). This study shows T. cruzi circulation in triatomines in the area of study, and because triatomines are adapted for living and reproducing in the domestic environment, there is a potential risk of Chagas disease transmission to humans. Also, we can conclude that the construction materials and house inhabitants are risk factors of triatomines infestation.