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Antimicrobial peptides are an essential component of the insect immune system. One of the most ubiquitous is defensin, which has been identified in all examined insect orders. Triatoma brasiliensis (Heteroptera, Triatominae), the main Trypanosoma cruzi vector in semi-arid regions of north-eastern Brazil, expresses def1, a defensin encoding gene, predominantly in the anterior region (cardia and stomach) of the midgut. In the present study, we compared the transcript abundance of T. brasiliensis def1 in the anterior (stomach) and posterior midgut (small intestine) regions of naive bugs with those infected with a familiar T. cruzi isolate. In the stomach, only slight differences between the two insect groups were visible, whereas in the small intestine wide differences (up to 9.6-fold) between infected and noninfected bugs become apparent. The highly increased def1 transcript abundance at 20 days after the infective blood meal might be a response to the T. cruzi infection, suggesting a potential function of intestinal defensin in the parasite population control.