The susceptibility to insecticides of triatomine species must be evaluated because of their involvement in the transmission of the Chagas disease. In each region with Chagas endemicity, evaluation of insecticide response is necessary to predict the success of the control campaigns.The baseline susceptibility was determined for the active principles deltamethrin, beta-cypermethrin and fenitrothion in nymphs of first and fifth instar of Triatoma dimidiata and nymphs of first instar of Triatoma maculata.The insecticide activity in triatomines was evaluated by the technique of topical application.The values of the LD50 in nymphs of first instar for T. maculata, expressed in nanograms per insect (ng/i), were 0.07, 0.05 and 4.12 for deltamethrin, beta-cypermethrin and fenitrothion respectively. The corresponding LD99 values were 1.08, 0.37 and 17.89 ng/i. In T. dimidiata, the LD50 values were 0.44, 0.46 and 16.45 ng/i; the LD99 values were 2.22, 1.97 and 36.07 ng/i. In nymphs of fifth instar T. dimidiata, the LD50 values were 510.7, 1623.6 and 838.9 ng/i; the LD99 values were 9607.5, 11,717.9 and 1525.0 ng/i, respectively.In first instar nymphs of T. dimidiata and T. maculata, the pyrethroid insecticides were more effective; in fifth instar nymphs of T. dimidiata, the effectiveness of the pyrethroids and the organophosphate differed in the LD50 comparison--the nymphs required much higher doses compared with the other triatomines and suggested a low susceptibility. The LD99 for the organophosphate (fenitrothion) was significantly lower and may indicate its greater effectiveness in field. Studies of synergistic effects amonst insecticides are important to clarify the role of biochemical mechanisms that determine tolerance to the pyrethroids. Insecticide tolerance represents a new challenge for control campaigns in the Andean and Central American countries where Chagas disease is endemic.