Responses of three different strains (resistance, susceptible and cross strains) of diamondback moth (DBM), Plutella xylostella L. on Bacillus thuringiensis var. aizawai (Bta) and fipronil were studied. Results showed that mortality of DBM larvae of all strains generally decrease with concentrations of the insecticides solutions. However, larval mortality seemed to be higher when treated with fipronil than with Bta. The $ extrm{LC}_{50}$ value for the susceptible strain treated with Bta (0.00083 ppm) was significantly lower than that of fipronil (0.00124ppm). In contrast, the $ extrm{LC}_{50}$ for resistance strain treated with Bta (0.03702 ppm) was significantly higher than with fipronil (0.00112 ppm), indicating that DBM populations of Karak farm have been exposed to Bta longer than to fipronil. Similarly, the cross strain treated with Bta had a significantly higher $ extrm{LC}_{50}$ value (0.00174 ppm) than that of fipronil (0.00078 ppm). This suggests that the mechanism of resistance of DBM larvae to Bta and fipronil is different and that it could be controlled by different set of gene loci. The use of Bta alternated with fipronil in the field with minimal cross-resistance development among DBM population is discussed.