Abstract: The 24h L C₅₀of parathion, isofenphos methy, monocrotophos and dichlorvos for Penaeus chinensis at its nauplis, zoaea, mysis and postlarvae stages is respectively 6.0, 4.0, 3.8 and 18×10^-3 mg·L^-1 ;8.0, 3.5, 3.5 and 3.0×10^-3 mg·L^-1 ;24, 24, 24 and 0.9mg·L^-1 and 48×10^-3, 48×10^-3, 48×10^-3 and 48×10^-3 mg·L^-1 ;and that of parathion for Penaeus vannamri at its same stages is 8.6, 8.2, 8.0 and 2.0×10^-3 mg·L^-1 respectively. The results show that the tolerance of prawn to thiono form organophosphorus pesticide is stronger at its early development stage than at other stages, but its sensitivity to oxo form organophosphours pesticide is almost the same at its all development stages.

Key words: Prawn, Organophosphorus pesticides, Sensitivity, plant growth promoting, Bacillus, microflora