The aim of the currrent investigation was to evaluate (a) the toxicity of three pyrethroids (deltamethrin, lambdacyhalothrin, and tetramethrin); (b) the effect of these insecticides on the locomotor activity; and (c) the repellent effect of N,N-diethyl-m-toluamide (DEET) on two deltamethrin-resistant strains of Triatoma infestans from Argentina (El Chorro and La Toma), and one susceptible strain. The resistance ratios (RRs) obtained for the La Toma strain were: > 10, 769, 50.7, and > 5.2 for

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... tamethrin, lambda-cyhalothrin, and tetramethrin respectively. The RRs for the El Chorro strain were: > 10, 769, 85.8, and > 5.2 for deltamethrin, lambda-cyhalothrin, and tetramethrin respectively. The hyperactivity usually caused by the three pyrethroids was in both the deltamethrin-resistant strains compared to the susceptible reference strain. No differences were observed in the repellent effect of DEET between the three groups. These results indicate that the deltamethrin-resistant insects have a cross resistance to lambdacyhalothrin and tetramethrin, and are also resistant to the first symptom of pyrethroid poisoning (hyperactivity). However, the sensorial process related to DEET repellency does not appear to be altered.