Insecticide Chlorpyrifos ethyl (CE) is common use for insect control in rice cultivation in Mekong Delta. Snakehead fish (Channa striata) often come to ricefield for spawning in rainy season. Therefore this species is high risk of pesticide exposure. This study aimed to assess effects of CE application for ricefield on snakehead and potential of applying enzyme re-activation techniques as biomarker of pesticide exposure. Fish was exposed into ricefield condition in Phung Hiep District – Hau Giang province for 7 days and then applied MONDEO 60EC as indication dose. Water was collected at day 7 before pesticide application and 1hr, 1, 3, 5, 7, and 14 d post spray for CE detection. Fish was sampled at day one prior spraying and day 1, 3, 5, 7 and 14 after spraying for ChE assay. Water concentration of CE after 1hr spraying varied between 1.28 – 7.07µg/L and below detection limit after 5 d post spraying. It was found of 13% mortality after one day application and more than 80% ChE inhibition at day 3 post spraying. ChE gradually recovered but up to day 14 it was still lower than pre-spraying. Re-activation techniques can be used for marking the effect of CE on ChE up to one week exposure as well as for monitoring pesticide contamination.