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The dynamics of vectorial transmission of Trypanosoma cruzi and the level of host (guinea pigs) protection after immunization with attenuated parasites (TCC strain) was studied under natural climatic conditions in an endemic region of northern Argentina. The experimental design included two guinea pig corrals isolated by mosquito netting. One (controls) had 17 healthy and susceptible adult guinea pigs. The other had 19 guinea pigs immunized with attenuated T. cruzi TCC strain. Each corral was colonized in April 1988 with equal-sized populations of Triatoma infestans naturally infected by T. cruzi. To evaluate relevant variables in the natural transmission of Chagas' disease, corrals were sampled in both winter and late spring to assess vector populations, and to carry out parasitologic studies on both vertebrate and invertebrate hosts. In both corrals, vector density decreased in winter and reached a maximum in the hot season. The vector infection rate was very high (> 50%) throughout the experiment. Vector infectivity increased with temperature and vector age, but did not differ between the experimental and control corrals. The vector-host contact rate showed a close relationship with temperature, although a very high vector density decreased this rate, even with high ambient temperatures. Initial infections by T. cruzi occurred among guinea pigs only during the hot season. Vectorial transmission risk was estimated from the total number of bug bites per day, the proportion of infected bugs, and the daily incidence in the guinea pig population. During the hot season, this risk was 6.84 × 10-4 in the control group and 1.82 × 10-4 in the immunized group.