Insecticide quinalphos is popularly used in Vietnam and detected in different water bodies, specially rice paddy fields where snakehead fish (Channa striata) often distribute. Therefore, the species can be at a high risk of exposure to insecticides. In this study, the effects of quinalphos on juvenile snakehead fish were assessed under both laboratory and field conditions. The 96h-LC50 was estimated by conduct in a static non-renewed system. Afterward, two quinalphos treatments (1% and 5% 96h-LC50) and a control were conducted for 96h to assess the sensitivity of brain cholinesterase of snakehead fish to quinalphos. Two rice-field treatments in the VMD were selected and then three cages were randomly placed into each rice field. One field was not applied with insecticide, and the other was applied Kinalux 25EC at once as an indication dose. The results showed that quinalphos is very toxic for snakehead fish (4-5g) with an 96h-LC50 of 49 µg/L. Quinalphos significantly inhibited the brain ChE activity of snakehead fish at a concentration of 0.49 µg/L (#1% 96hLC50). Using Kinalux 25EC for rice at the instructed dose causes serious effects for snakehead fish, including high mortality and high and prolonged ChE inhibition. Although quinalphos was below the detection limit (0.1 µg/L) after 1 d post-spray, activity of brain ChE of the fish species was significantly inhibited up to a week post-spray. Residues of quinalphos in ricefield soil and this species should be concentrated to determine in further study for better understanding the fate of this insecticide after spraying and to assess the safety of snakehead fish meat for food.