Diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae), has evolved resistance to various kinds of insecticides in the field. Chlorantraniliprole, which is a new pesticide of the anthranilic diamide group, provides potent and broad-spectrum activity within the insect order Lepidoptera. This insecticide provides a new option for control of P. xylostella populations resistant to other chemicals. The susceptibility of 16 field populations and seven laboratory maintained strains of P. xylostella to chlorantraniliprole were determined through leaf dip bioassay. The susceptibility variation among 16 field populations was low (five-fold), with median lethal concentrations (LC₅₀ values) varying from 0.221 to 1.104 mg/liter. However, wider ranges of variation in LC₅₀ values (10-fold) were observed among seven laboratory strains. Low level tolerance (six- to 10-fold) was detected in two laboratory-selected strains and three field-collected populations when compared with the susceptible Roth strain. A discriminating concentration (15 mg/liter) was calibrated from pooled toxicological data of the 16 field populations, as an important first step in resistance management, for the routine monitoring of resistance in the future. When assessed at the established discriminating dose 15 mg/liter, seven laboratory strains and five field populations showed an average mortality of 99.75% (from 98 to100%). Synergism assays showed metabolic enzymes might be involved in chlorantraniliprole detoxification in the susceptible Roth strain, but not in the additional observed tolerance of strains selected for resistance with other insecticides.