The biological susceptibility of Triatoma sordida and Triatoma infestans to deltamethrin and lambdacyhalothrin was evaluated in three homogeneous groups of houses in the municipality of Posse, Goiás. Each group of houses was treated according to one of the following protocols: deltamethrin 5% wetted powder at 25 mg a.i./m2; deltamethrin 5% concentrated suspension at 25 mg a.i./m2; and lambdacyhalothrin 10% wetted powder at 30 mg a.i./m2. Monitoring of susceptibility to the insecticides was done after 90, 210, 270 and 360 days after the spraying. Each test consisted of an exposure of 10 third-instar nymphs of each species for every house. The insects were kept in contact with the sprayed wall for a period of 72 hours in plastic containers developed by WHO for studies with adult mosquitos. Thirty days after the exposure the death numbers were scored. Results demonstrate persistent activity of the three insecticides for up to a year after spraying and equal biological susceptibilities for both Triatoma sordida and Triatoma infestans to each formulation.