The diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae), is considered as one of the most difficult pests to control. It has developed resistance not only to synthetic insecticides but also to Bacillus thuringiensis-based pesticides. We tested the hypothesis that selection in a P. xylostella population, from Hosur, India, with deltamethrin would give a broad spectrum of resistance to several insecticides. We also were interested in genetically classifying resistance to deltamethrin in the selected population and in evaluating whether resistance can be suppressed using synergists. Bioassays (at generation 1, G1) using deltamethrin indicated a resistance ratio of 161-fold compared with a laboratory-susceptible population of P. xylostella (Lab-UK). At G2, the field-derived population was divided into two subpopulations; one population was selected (G1 to G8) with deltamethrin (Delta-SEL), and the second population was left unselected (UNSEL). Bioassays at G9 indicated that selection with deltamethrin gave a resistance ratio of 15-fold compared with UNSEL and 1,647-fold compared with Lab-UK. The resistance to deltamethrin in the UNSEL population was stable. The Delta-SEL population maintained resistance to lambda-cyhalothrin, but there was no cross-resistance to indoxacarb, DDT, or Cry1Ac. Crossing experiments indicated that resistance to deltamethrin in Delta-SEL was multigenic and inherited in an incompletely dominant fashion. Piperonyl butoxide (PBO) and S.S.S-tri-n-butyl phosphorotrithioate with potent inhibitory activity against esterases and/or monooxygenases significantly increased the toxicity of deltamethrin against both UNSEL and Delta-SEL, but they showed no such synergism with Lab-UK. Thus, it can be predicted that development of resistance to deltamethrin would be delayed under appropriate control strategies that favor the dilution of resistance alleles by enhanced flow of susceptible alleles. Further analysis suggested that mixing PBO and deltamethrin could eliminate the substantial resistance to deltamethrin in this population.