Pesticides containing diazinon are frequently used in agriculture in the Vietnamese Mekong delta region leading
to their potential residual occurrence in the environment. Under laboratory conditions, exposure to diazinon has
been shown to result in adverse inhibition of cholinesterase enzyme (ChE) activity and subsequent death in
several fish species. This study investigated a 96-h median lethal concentration (LC50) of diazinon concentrations
from 0.08 to 1.25 mg/L on juvenile giant freshwater prawns (Macrobrachium rosenbergii de Man, 1879) in
tanks in the laboratory. Inhibition of ChE in the flesh and in the eyes of the tested shrimps after exposed to
diazinon concentrations of 2.7, 27, 67.5 μg/L equivalent to 1%, 10%, 25% LC50–96 h was calculated. The results
indicated that diazinon was highly toxic to giant freshwater prawn with a low LC50–96 h of 270 μg/L. The
activity of ChE in the flesh was more sensitive to diazinon than that in the eyes. Furthermore, in the future, the
activity of ChE in the flesh or in the eyes of shrimps has potential to be used as biomarker for rapid recognition of
diazinon contamination in water.