Bioassays (at generation G₂) with a newly collected field population (designated MN) of Plutella xylostella (L.) (Lepidoptera: Plutellidae) from Multan, Pakistan, indicated resistance to spinosad, indoxacarb, deltamethrin, abamectin, and acetamiprid. At G₂ the field-derived population was divided into two subpopulations, one was selected (G₂ to G₁₁) with spinosad (Spino-SEL), whereas the second was left unselected (UNSEL). A significant reduction in the resistance ratio for each compound was observed in UNSEL at G₁₂, indicating that the observed resistance to each insecticide was unstable. For Spino-SEL, bioassays at G₁₂ found that selection with spinosad gave a resistance ratio of 283 compared with MN at G₂. The resistance to indoxacarb and acetamiprid in the Spino-SEL population increased to 13- and 67-fold, respectively, compared with MN at G_2. The toxicity of deltamethrin to Spino-SEL was similar to its toxicity to the MN population at G₂. This suggests that spinosad selection maintained the otherwise unstable resistance to the compound. In contrast, resistance to abamectin decreased significantly from G₂ to G₁₂ in Spino-SEL. Logit regression analysis of F₁ reciprocal crosses between Spino-SEL and the susceptible Lab-UK indicated that resistance to spinosad was inherited as an autosomal, incompletely recessive trait. The spinosad resistance allele significantly delays the developmental time, reduced pupal weight, number of eggs laid, and number of eggs hatched compared with Lab-UK. Further analysis suggests Spino-SEL exhibited a significantly lower intrinsic rate of population increase (r_m) to all other populations tested.