Bioassays (at generation G₂) with a newly collected field population (designated CH4) ofPlutella xylostella (L.) (Lepidoptera: Plutellidae) from farms in the Cameron Highlands, Malaysia, indicated resistance to acetamiprid, deltamethrin, lambda-cyhalothrin, and esfenvalerate. At G₃, the field-derived population was divided into two subpopulations, one subpopuplation was selected (G₃ to G₁₀) with acetamiprid (aceta-SEL), whereas the second subpopulation was left unselected (UNSEL). A significant reduction in the resistance ratio for each compound was observed in UNSEL at G₁₁, indicating that the observed resistance to each insecticide was unstable. For aceta-SEL, bioassays at G₁₁ found that selection with acetamiprid gave a resistance ratio of 409 compared with UNSEL. The LC₅₀ values for deltamethrin, lambda-cyhalothrin, and esfenvalerate to aceta-SEL were similar at both G₁₁ and G₂. This suggests that acetamiprid selection maintained the otherwise unstable resistance to these compounds in the aceta-SEL population. Logit regression analysis of F₁ reciprocal crosses between aceta-SEL and the susceptible Lab-UK indicated that resistance to acetamiprid was inherited as an autosomal, incompletely recessive (D_LC = 0.26) trait. Tests of monogenic inheritance and weight distribution suggested that resistance to acetamiprid was controlled by a single locus.