The enzymatic effect of locust tissues upon hydrolysis of the fluorinated Δ2-oxazoline-1,3 Ia was elucidated using 19F[1H] NMR monitoring.In a phosphate buffer at pH=7.4 (mean physiological pH of locust tissues), the substrate Ia hydrolyses slowly into the corresponding fluorinated hydroxylamide VIa.If diluted, locust haemolymph (12.5% in phosphate buffer) catalyses slightly this hydrolytic pathway, it overall triggers the unmasking of carboxylate IIIa, corresponding to the expected proinsecticide behaviour of Ia. This behaviour is spectacularly almost the unique reaction observed during in vitro assays in concentrated fat body and mesenteron. Inasmuch as β-hydroxylamide VIa is not hydrolysed into carboxylate IIIa during such conditions, it must be concluded that carboxylate formation exclusively results from hydration and hydrolysis of substrate Ia via the aminoester Va. The formation of this intermediate aminoester is demonstrated by complementary assays. The enzymes supposed to intervene are of the α-chymotrypsine type for the first step (hydration) and of the esterase type for subsequent hydrolysis of intermediate aminoester Va.Thus, this work constitutes the first example of a Δ2-oxazoline-1,3 structure exploited for elaborating proinsecticides of carboxylates III and/or β-ethanolamines II based on enzymatic activation in insects.