Laboratory assays were conducted to assess the potential of pymetrozine, a novel azomethine pyridine insecticide, against the brown planthopper, Nilaparvata lugens (Stål) (Homoptera: Delphacidae). All nymphal instars were treated with the pymetrozine concentrations of 400, 200, 100, 50, 25 mg/l, and then maintained in incubators at 26 ± 1°C on a photoperiod of 16:8 h (L: D). At 24, 48, 72, and 120 h after treatment, mortality was recorded. Greater numbers of nymphs were killed at a faster rate with the higher concentrations (mortality of 70.7% with 400 mg/l and 63.8% with 200 mg/l at 24 h, first -second) than at the median concentrations (mortality of 51.7% with 100 mg/l and 50% with 50 mg/l at 24 h, first -second) and the lower concentration (mortality of 25.9% with 25 mg/l at 24 h, first -second), whereas mortality in the control remained low (3.3% with distilled water at 24 h, first -second). The first and second instars were the most susceptible with median lethal concentrations [LC₅₀] of 76.3, 35.5, 26.6 and 21.96 mg/L at 24, 48, 72 and 120 h after treatment, respectively. Fifth-instars were the least susceptible with the LC₅₀ values of 5,887.53, 758.41, 236.15 and 67.23 mg/L at 24, 48, 72 and 120 h after treatment, respectively. The LC₅₀s of the third - fourth instars were 1737.89, 601.6, 96.21 and 50.14 mg/L at 24, 48, 72 and 120 h after treatment, respectively, and were significantly greater than those of the first - second instars. These results indicate that pymetrozine would be an effective alternative for the control of brown planthopper.