Abstract: Laboratory bioassay of Bacillus thuringiensis (Bt) were conducted using larva e of the diamond back moth(DBM), Plutella xylostela fed with vegetable leaves of various growth stages.The leaves were dipped in the Bt solution.The results showed that the difference in the growth stage of the vegetable leaves had a sig nificant effect on the LC ₅₀ of the Bt solution to DBM.The LC ₅₀ obtai ned with leaves picked at the peak vegetative growth stage was 4 times higher th an that with leaves of the late vegetative growth stage.It is thus important to use plant material of standard growth stage in the bioassay for Bt preparations.