The susceptibility of early instars of diamondback moth, Plutella xylostella (L.), and early (first and second) and late instars (third and fourth) of cabbage looper, Trichoplusia ni (Hübner), from Minnesota and south Texas, to indoxacarb and λ-cyhalothrin was determined in the laboratory. Susceptibilities of the two species from the two geographical locations to indoxacarb and λ-cyhalothrin varied greatly. P. xylostella from Minnesota was as susceptible to indoxacarb as those from south Texas, whereas both early and late instar T. ni from south Texas were significantly more tolerant to indoxacarb than those from Minnesota. The LC₅₀ values of indoxacarb for early and later instar T. ni at 48 h from south Texas were 4.3- and 34.0-fold greater than those from Minnesota, respectively. Similarly, early instar P. xylostella and late instar T. ni from south Texas were significantly less susceptible to λ-cyhalothrin than those from Minnesota. Percentage mortality of the two insect species caused by the two insecticides varied with time of exposure and generally exhibited similar patterns of responses to different concentrations for each insecticide.